Real Estate Investments And Accumulated Depreciation (Schedule Of Real Estate Investments And Accumulated Depreciation) (Details)

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Real Estate Investments And Accumulated Depreciation (Schedule Of Real Estate Investments And Accumulated Depreciation) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Related Encumbrances	$ 756,910,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances
Initial Costs, Land	778,751,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand
Initial Costs, Building and Improvements	3,510,501,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements
Costs Capitalized Subsequent to Acquisition	668,927,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts
Gross Amount at Which Carried at Close of Period, Land	760,855,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,197,324,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements
Total	4,958,179,000us-gaap_RealEstateGrossAtCarryingValue		5,129,933,000us-gaap_RealEstateGrossAtCarryingValue	5,379,436,000us-gaap_RealEstateGrossAtCarryingValue	5,279,770,000us-gaap_RealEstateGrossAtCarryingValue
Accumulated depreciation	1,414,305,000us-gaap_RealEstateAccumulatedDepreciation		1,400,988,000us-gaap_RealEstateAccumulatedDepreciation	1,478,214,000us-gaap_RealEstateAccumulatedDepreciation	1,409,163,000us-gaap_RealEstateAccumulatedDepreciation
Real Estate Aggregate Cost, Tax Purpose	3,200,000,000cli_RealEstateAggregateCostTaxPurpose
Office [Member] | One Bridge Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	1996
Initial Costs, Land	2,439,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBridgePlazaMember
Initial Costs, Building and Improvements	24,462,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBridgePlazaMember
Costs Capitalized Subsequent to Acquisition	7,701,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBridgePlazaMember
Gross Amount at Which Carried at Close of Period, Land	2,439,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBridgePlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	32,163,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBridgePlazaMember	[1]
Total	34,602,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBridgePlazaMember
Accumulated depreciation	14,550,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBridgePlazaMember	[2]
Office [Member] | 2115 Linwood Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	1998
Initial Costs, Land	474,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoThousandOneHundredFifteenLinwoodAvenueMember
Initial Costs, Building and Improvements	4,419,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoThousandOneHundredFifteenLinwoodAvenueMember
Costs Capitalized Subsequent to Acquisition	5,870,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoThousandOneHundredFifteenLinwoodAvenueMember
Gross Amount at Which Carried at Close of Period, Land	474,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoThousandOneHundredFifteenLinwoodAvenueMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	10,289,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoThousandOneHundredFifteenLinwoodAvenueMember	[1]
Total	10,763,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoThousandOneHundredFifteenLinwoodAvenueMember
Accumulated depreciation	3,629,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoThousandOneHundredFifteenLinwoodAvenueMember	[2]
Office [Member] | 210 Clay Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1978
Acquired	2009
Related Encumbrances	13,330,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTenClayAvenueMember
Initial Costs, Land	2,300,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTenClayAvenueMember
Initial Costs, Building and Improvements	11,189,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTenClayAvenueMember
Costs Capitalized Subsequent to Acquisition	(9,034,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTenClayAvenueMember
Gross Amount at Which Carried at Close of Period, Land	781,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTenClayAvenueMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,674,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTenClayAvenueMember	[1]
Total	4,455,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTenClayAvenueMember
Accumulated depreciation	771,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTenClayAvenueMember	[2]
Office [Member] | 135 Chestnut Ridge Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	1997
Initial Costs, Land	2,587,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThirtyFiveChestnutRidgeRoadMember
Initial Costs, Building and Improvements	10,350,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThirtyFiveChestnutRidgeRoadMember
Costs Capitalized Subsequent to Acquisition	1,783,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThirtyFiveChestnutRidgeRoadMember
Gross Amount at Which Carried at Close of Period, Land	2,588,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThirtyFiveChestnutRidgeRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	12,132,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThirtyFiveChestnutRidgeRoadMember	[1]
Total	14,720,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThirtyFiveChestnutRidgeRoadMember
Accumulated depreciation	5,104,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThirtyFiveChestnutRidgeRoadMember	[2]
Office [Member] | 15 East Midland Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1997
Related Encumbrances	12,615,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenEastMidlandAvenueMember
Initial Costs, Land	10,375,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenEastMidlandAvenueMember
Initial Costs, Building and Improvements	41,497,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenEastMidlandAvenueMember
Costs Capitalized Subsequent to Acquisition	2,388,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenEastMidlandAvenueMember
Gross Amount at Which Carried at Close of Period, Land	10,374,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenEastMidlandAvenueMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	43,886,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenEastMidlandAvenueMember	[1]
Total	54,260,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenEastMidlandAvenueMember
Accumulated depreciation	18,222,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenEastMidlandAvenueMember	[2]
Office [Member] | 140 East Ridgewood [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	1997
Related Encumbrances	11,887,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyEastRidgewoodAvenueMember
Initial Costs, Land	7,932,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyEastRidgewoodAvenueMember
Initial Costs, Building and Improvements	31,463,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyEastRidgewoodAvenueMember
Costs Capitalized Subsequent to Acquisition	7,012,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyEastRidgewoodAvenueMember
Gross Amount at Which Carried at Close of Period, Land	7,932,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyEastRidgewoodAvenueMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	38,475,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyEastRidgewoodAvenueMember	[1]
Total	46,407,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyEastRidgewoodAvenueMember
Accumulated depreciation	16,309,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyEastRidgewoodAvenueMember	[2]
Office [Member] | 461 From Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1997
Initial Costs, Land	13,194,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredSixtyOneFromRoadMember
Initial Costs, Building and Improvements	52,778,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredSixtyOneFromRoadMember
Costs Capitalized Subsequent to Acquisition	4,233,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredSixtyOneFromRoadMember
Gross Amount at Which Carried at Close of Period, Land	13,194,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredSixtyOneFromRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	57,011,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredSixtyOneFromRoadMember	[1]
Total	70,205,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredSixtyOneFromRoadMember
Accumulated depreciation	22,956,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredSixtyOneFromRoadMember	[2]
Office [Member] | 650 From Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1978
Acquired	1997
Related Encumbrances	22,804,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyFromRoadMember
Initial Costs, Land	10,487,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyFromRoadMember
Initial Costs, Building and Improvements	41,949,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyFromRoadMember
Costs Capitalized Subsequent to Acquisition	7,984,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyFromRoadMember
Gross Amount at Which Carried at Close of Period, Land	10,487,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyFromRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	49,933,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyFromRoadMember	[1]
Total	60,420,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyFromRoadMember
Accumulated depreciation	20,867,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyFromRoadMember	[2]
Office [Member] | 61 South Paramus Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985	[3]
Acquired	1997	[3]
Related Encumbrances	22,319,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyOneSouthParamusRoadMember	[3]
Initial Costs, Land	9,005,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyOneSouthParamusRoadMember	[3]
Initial Costs, Building and Improvements	36,018,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyOneSouthParamusRoadMember	[3]
Costs Capitalized Subsequent to Acquisition	7,913,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyOneSouthParamusRoadMember	[3]
Gross Amount at Which Carried at Close of Period, Land	9,005,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyOneSouthParamusRoadMember	[1],[3]
Gross Amount at Which Carried at Close of Period, Building and Improvements	43,931,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyOneSouthParamusRoadMember	[1],[3]
Total	52,936,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyOneSouthParamusRoadMember	[3]
Accumulated depreciation	19,029,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyOneSouthParamusRoadMember	[2],[3]
Office [Member] | 120 West Passaic Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1972
Acquired	1997
Initial Costs, Land	1,354,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyWestPassaicStreetMember
Initial Costs, Building and Improvements	5,415,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyWestPassaicStreetMember
Costs Capitalized Subsequent to Acquisition	431,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyWestPassaicStreetMember
Gross Amount at Which Carried at Close of Period, Land	1,357,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyWestPassaicStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,843,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyWestPassaicStreetMember	[1]
Total	7,200,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyWestPassaicStreetMember
Accumulated depreciation	2,424,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyWestPassaicStreetMember	[2]
Office [Member] | 365 West Passaic Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1976
Acquired	1997
Related Encumbrances	11,887,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSixtyFiveWestPassaicStreetMember
Initial Costs, Land	4,148,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSixtyFiveWestPassaicStreetMember
Initial Costs, Building and Improvements	16,592,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSixtyFiveWestPassaicStreetMember
Costs Capitalized Subsequent to Acquisition	4,442,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSixtyFiveWestPassaicStreetMember
Gross Amount at Which Carried at Close of Period, Land	4,148,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSixtyFiveWestPassaicStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	21,034,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSixtyFiveWestPassaicStreetMember	[1]
Total	25,182,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSixtyFiveWestPassaicStreetMember
Accumulated depreciation	9,216,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSixtyFiveWestPassaicStreetMember	[2]
Office [Member] | 395 West Passaic Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1979
Acquired	2006
Initial Costs, Land	2,550,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyFiveWestPassaicStreetMember
Initial Costs, Building and Improvements	17,131,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyFiveWestPassaicStreetMember
Costs Capitalized Subsequent to Acquisition	437,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyFiveWestPassaicStreetMember
Gross Amount at Which Carried at Close of Period, Land	2,550,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyFiveWestPassaicStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	17,568,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyFiveWestPassaicStreetMember	[1]
Total	20,118,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyFiveWestPassaicStreetMember
Accumulated depreciation	3,717,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyFiveWestPassaicStreetMember	[2]
Office [Member] | 1 Lake Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1994
Acquired	1997
Related Encumbrances	40,756,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneLakeStreetMember
Initial Costs, Land	13,952,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneLakeStreetMember
Initial Costs, Building and Improvements	55,812,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneLakeStreetMember
Costs Capitalized Subsequent to Acquisition	(37,797,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneLakeStreetMember
Gross Amount at Which Carried at Close of Period, Land	6,268,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneLakeStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	25,699,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneLakeStreetMember	[1]
Total	31,967,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneLakeStreetMember
Accumulated depreciation	10,336,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneLakeStreetMember	[2]
Office [Member] | 10 Mountainview Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1986
Acquired	1998
Initial Costs, Land	4,240,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenMountainviewRoadMember
Initial Costs, Building and Improvements	20,485,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenMountainviewRoadMember
Costs Capitalized Subsequent to Acquisition	4,480,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenMountainviewRoadMember
Gross Amount at Which Carried at Close of Period, Land	4,240,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenMountainviewRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	24,965,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenMountainviewRoadMember	[1]
Total	29,205,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenMountainviewRoadMember
Accumulated depreciation	10,456,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenMountainviewRoadMember	[2]
Office [Member] | 400 Chestnut Ridge Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1982
Acquired	1997
Initial Costs, Land	4,201,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredChestnutRidgeRoadMember
Initial Costs, Building and Improvements	16,802,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredChestnutRidgeRoadMember
Costs Capitalized Subsequent to Acquisition	(6,650,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredChestnutRidgeRoadMember
Gross Amount at Which Carried at Close of Period, Land	2,312,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredChestnutRidgeRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	12,041,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredChestnutRidgeRoadMember	[1]
Total	14,353,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredChestnutRidgeRoadMember
Accumulated depreciation	6,431,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredChestnutRidgeRoadMember	[2]
Office [Member] | 50 Tice Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	1994
Related Encumbrances	23,289,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTiceBoulevardMember
Initial Costs, Land	4,500,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTiceBoulevardMember
Costs Capitalized Subsequent to Acquisition	26,578,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTiceBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	4,500,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTiceBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	26,578,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTiceBoulevardMember	[1]
Total	31,078,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTiceBoulevardMember
Accumulated depreciation	18,326,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTiceBoulevardMember	[2]
Office [Member] | 300 Tice Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1991
Acquired	1996
Initial Costs, Land	5,424,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTiceBoulevardMember
Initial Costs, Building and Improvements	29,688,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTiceBoulevardMember
Costs Capitalized Subsequent to Acquisition	5,970,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTiceBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	5,424,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTiceBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	35,658,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTiceBoulevardMember	[1]
Total	41,082,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTiceBoulevardMember
Accumulated depreciation	15,466,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTiceBoulevardMember	[2]
Office [Member] | 150 J.F. Kennedy Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1980
Acquired	1997
Initial Costs, Land	12,606,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyJFKennedyParkwayMember
Initial Costs, Building and Improvements	50,425,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyJFKennedyParkwayMember
Costs Capitalized Subsequent to Acquisition	8,846,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyJFKennedyParkwayMember
Gross Amount at Which Carried at Close of Period, Land	12,606,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyJFKennedyParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	59,271,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyJFKennedyParkwayMember
Total	71,877,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyJFKennedyParkwayMember
Accumulated depreciation	24,979,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyJFKennedyParkwayMember	[2]
Office [Member] | 4 Becker Farm Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	2009
Related Encumbrances	39,421,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourBeckerFarmRoadMember
Initial Costs, Land	5,600,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourBeckerFarmRoadMember
Initial Costs, Building and Improvements	38,285,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourBeckerFarmRoadMember
Costs Capitalized Subsequent to Acquisition	(9,054,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourBeckerFarmRoadMember
Gross Amount at Which Carried at Close of Period, Land	4,271,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourBeckerFarmRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	30,560,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourBeckerFarmRoadMember
Total	34,831,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourBeckerFarmRoadMember
Accumulated depreciation	5,854,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourBeckerFarmRoadMember	[2]
Office [Member] | 5 Becker Farm Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1982
Acquired	2009
Related Encumbrances	13,867,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveBeckerFarmRoadMember
Initial Costs, Land	2,400,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveBeckerFarmRoadMember
Initial Costs, Building and Improvements	11,885,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveBeckerFarmRoadMember
Costs Capitalized Subsequent to Acquisition	(5,626,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveBeckerFarmRoadMember
Gross Amount at Which Carried at Close of Period, Land	1,492,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveBeckerFarmRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,167,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveBeckerFarmRoadMember
Total	8,659,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveBeckerFarmRoadMember
Accumulated depreciation	1,128,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveBeckerFarmRoadMember	[2]
Office [Member] | 6 Becker Farm Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	2009
Related Encumbrances	14,226,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixBeckerFarmRoadMember
Initial Costs, Land	2,600,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixBeckerFarmRoadMember
Initial Costs, Building and Improvements	15,548,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixBeckerFarmRoadMember
Costs Capitalized Subsequent to Acquisition	(4,667,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixBeckerFarmRoadMember
Gross Amount at Which Carried at Close of Period, Land	1,883,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixBeckerFarmRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	11,598,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixBeckerFarmRoadMember
Total	13,481,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixBeckerFarmRoadMember
Accumulated depreciation	2,113,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixBeckerFarmRoadMember	[2]
Office [Member] | 101 Eisenhower Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1980
Acquired	1994
Initial Costs, Land	228,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneEisenhowerParkwayMember
Costs Capitalized Subsequent to Acquisition	21,542,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneEisenhowerParkwayMember
Gross Amount at Which Carried at Close of Period, Land	228,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneEisenhowerParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	21,542,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneEisenhowerParkwayMember
Total	21,770,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneEisenhowerParkwayMember
Accumulated depreciation	12,971,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneEisenhowerParkwayMember	[2]
Office [Member] | 103 Eisenhower Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	1994
Costs Capitalized Subsequent to Acquisition	16,519,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeEisenhowerParkwayMember
Gross Amount at Which Carried at Close of Period, Land	2,300,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeEisenhowerParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	14,219,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeEisenhowerParkwayMember
Total	16,519,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeEisenhowerParkwayMember
Accumulated depreciation	8,961,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeEisenhowerParkwayMember	[2]
Office [Member] | 105 Eisenhower Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2001
Acquired	2001
Initial Costs, Land	4,430,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiveEisenhowerParkwayMember
Initial Costs, Building and Improvements	42,898,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiveEisenhowerParkwayMember
Costs Capitalized Subsequent to Acquisition	5,289,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiveEisenhowerParkwayMember
Gross Amount at Which Carried at Close of Period, Land	3,835,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiveEisenhowerParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	48,782,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiveEisenhowerParkwayMember
Total	52,617,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiveEisenhowerParkwayMember
Accumulated depreciation	22,007,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiveEisenhowerParkwayMember	[2]
Office [Member] | 75 Livingston Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	2009
Related Encumbrances	10,888,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveLivingstonAvenueMember
Initial Costs, Land	1,900,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveLivingstonAvenueMember
Initial Costs, Building and Improvements	6,312,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveLivingstonAvenueMember
Costs Capitalized Subsequent to Acquisition	(1,637,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveLivingstonAvenueMember
Gross Amount at Which Carried at Close of Period, Land	1,281,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveLivingstonAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,294,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveLivingstonAvenueMember
Total	6,575,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveLivingstonAvenueMember
Accumulated depreciation	1,045,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveLivingstonAvenueMember	[2]
Office [Member] | 85 Livingston Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	2009
Related Encumbrances	15,243,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveLivingstonAvenueMember
Initial Costs, Land	2,500,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveLivingstonAvenueMember
Initial Costs, Building and Improvements	14,238,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveLivingstonAvenueMember
Costs Capitalized Subsequent to Acquisition	(6,542,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveLivingstonAvenueMember
Gross Amount at Which Carried at Close of Period, Land	1,471,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveLivingstonAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,725,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveLivingstonAvenueMember
Total	10,196,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveLivingstonAvenueMember
Accumulated depreciation	1,614,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveLivingstonAvenueMember	[2]
Office [Member] | Harborside Plaza 1 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	1996
Initial Costs, Land	3,923,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaOneMember
Initial Costs, Building and Improvements	51,013,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaOneMember
Costs Capitalized Subsequent to Acquisition	27,926,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaOneMember
Gross Amount at Which Carried at Close of Period, Land	3,923,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaOneMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	78,939,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaOneMember
Total	82,862,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaOneMember
Accumulated depreciation	36,168,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaOneMember	[2]
Office [Member] | Harborside Plaza 2 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1996
Initial Costs, Land	17,655,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaTwoMember
Initial Costs, Building and Improvements	101,546,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaTwoMember
Costs Capitalized Subsequent to Acquisition	21,449,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaTwoMember
Gross Amount at Which Carried at Close of Period, Land	12,844,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaTwoMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	127,806,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaTwoMember
Total	140,650,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaTwoMember
Accumulated depreciation	54,391,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaTwoMember	[2]
Office [Member] | Harborside Plaza 3 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1996
Initial Costs, Land	17,655,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaThreeMember
Initial Costs, Building and Improvements	101,878,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaThreeMember
Costs Capitalized Subsequent to Acquisition	21,116,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaThreeMember
Gross Amount at Which Carried at Close of Period, Land	12,843,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaThreeMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	127,806,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaThreeMember
Total	140,649,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaThreeMember
Accumulated depreciation	54,391,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaThreeMember	[2]
Office [Member] | Harborside Plaza 4A [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2000
Acquired	2000
Initial Costs, Land	1,244,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFouraMember
Initial Costs, Building and Improvements	56,144,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFouraMember
Costs Capitalized Subsequent to Acquisition	14,698,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFouraMember
Gross Amount at Which Carried at Close of Period, Land	1,244,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFouraMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	70,842,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFouraMember
Total	72,086,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFouraMember
Accumulated depreciation	28,181,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFouraMember	[2]
Office [Member] | Harborside Plaza 5 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2002
Acquired	2002
Related Encumbrances	221,563,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFiveMember
Initial Costs, Land	6,218,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFiveMember
Initial Costs, Building and Improvements	170,682,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFiveMember
Costs Capitalized Subsequent to Acquisition	52,013,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFiveMember
Gross Amount at Which Carried at Close of Period, Land	5,705,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFiveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	223,208,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFiveMember
Total	228,913,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFiveMember
Accumulated depreciation	80,254,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_HarborsidePlazaFiveMember	[2]
Office [Member] | 101 Hudson Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1992
Acquired	2005
Initial Costs, Land	45,530,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneHudsonStreetMember
Initial Costs, Building and Improvements	271,376,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneHudsonStreetMember
Costs Capitalized Subsequent to Acquisition	921,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneHudsonStreetMember
Gross Amount at Which Carried at Close of Period, Land	45,530,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneHudsonStreetMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	272,297,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneHudsonStreetMember
Total	317,827,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneHudsonStreetMember
Accumulated depreciation	68,400,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneHudsonStreetMember	[2]
Office [Member] | 3 AAA Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	2007
Initial Costs, Land	242,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeAaaDriveMember
Initial Costs, Building and Improvements	3,218,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeAaaDriveMember
Costs Capitalized Subsequent to Acquisition	1,598,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeAaaDriveMember
Gross Amount at Which Carried at Close of Period, Land	242,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeAaaDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,816,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeAaaDriveMember
Total	5,058,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeAaaDriveMember
Accumulated depreciation	1,438,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeAaaDriveMember	[2]
Office [Member] | 700 Horizon Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2007
Acquired	2007
Initial Costs, Land	490,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredHorizonDriveMember
Initial Costs, Building and Improvements	43,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredHorizonDriveMember
Costs Capitalized Subsequent to Acquisition	16,572,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Land	865,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	16,240,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredHorizonDriveMember
Total	17,105,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredHorizonDriveMember
Accumulated depreciation	3,519,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredHorizonDriveMember	[2]
Office [Member] | 2 South Gold Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1974
Acquired	2007
Initial Costs, Land	476,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSouthGoldDriveMember
Initial Costs, Building and Improvements	3,487,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSouthGoldDriveMember
Costs Capitalized Subsequent to Acquisition	839,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSouthGoldDriveMember
Gross Amount at Which Carried at Close of Period, Land	476,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSouthGoldDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,326,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSouthGoldDriveMember
Total	4,802,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSouthGoldDriveMember
Accumulated depreciation	813,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSouthGoldDriveMember	[2]
Office [Member] | 103 Carnegie Center [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	1996
Initial Costs, Land	2,566,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeCarnegieCenterMember
Initial Costs, Building and Improvements	7,868,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeCarnegieCenterMember
Costs Capitalized Subsequent to Acquisition	3,183,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeCarnegieCenterMember
Gross Amount at Which Carried at Close of Period, Land	2,566,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeCarnegieCenterMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	11,051,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeCarnegieCenterMember
Total	13,617,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeCarnegieCenterMember
Accumulated depreciation	5,072,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredThreeCarnegieCenterMember	[2]
Office [Member] | 2 Independence Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	2009
Initial Costs, Land	1,300,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoIndependenceWayMember
Initial Costs, Building and Improvements	7,246,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoIndependenceWayMember
Costs Capitalized Subsequent to Acquisition	126,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoIndependenceWayMember
Gross Amount at Which Carried at Close of Period, Land	1,300,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoIndependenceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,372,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoIndependenceWayMember
Total	8,672,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoIndependenceWayMember
Accumulated depreciation	1,392,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoIndependenceWayMember	[2]
Office [Member] | 3 Independence Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	1997
Initial Costs, Land	1,997,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeIndependenceWayMember
Initial Costs, Building and Improvements	11,391,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeIndependenceWayMember
Costs Capitalized Subsequent to Acquisition	4,186,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeIndependenceWayMember
Gross Amount at Which Carried at Close of Period, Land	1,997,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeIndependenceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	15,577,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeIndependenceWayMember
Total	17,574,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeIndependenceWayMember
Accumulated depreciation	6,986,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeIndependenceWayMember	[2]
Office [Member] | 100 Overlook Center [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1997
Initial Costs, Land	2,378,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOverlookCenterMember
Initial Costs, Building and Improvements	21,754,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOverlookCenterMember
Costs Capitalized Subsequent to Acquisition	3,384,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOverlookCenterMember
Gross Amount at Which Carried at Close of Period, Land	2,378,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOverlookCenterMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	25,138,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOverlookCenterMember
Total	27,516,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOverlookCenterMember
Accumulated depreciation	10,295,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOverlookCenterMember	[2]
Office [Member] | 5 Vaughn Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1995
Initial Costs, Land	657,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveVaughnDriveMember
Initial Costs, Building and Improvements	9,800,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveVaughnDriveMember
Costs Capitalized Subsequent to Acquisition	3,001,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveVaughnDriveMember
Gross Amount at Which Carried at Close of Period, Land	657,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveVaughnDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	12,801,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveVaughnDriveMember
Total	13,458,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveVaughnDriveMember
Accumulated depreciation	6,434,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveVaughnDriveMember	[2]
Office [Member] | 377 Summerhill Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1977
Acquired	1997
Initial Costs, Land	649,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSeventySevenSummerhillRoadMember
Initial Costs, Building and Improvements	2,594,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSeventySevenSummerhillRoadMember
Costs Capitalized Subsequent to Acquisition	412,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSeventySevenSummerhillRoadMember
Gross Amount at Which Carried at Close of Period, Land	649,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSeventySevenSummerhillRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,006,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSeventySevenSummerhillRoadMember
Total	3,655,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSeventySevenSummerhillRoadMember
Accumulated depreciation	1,355,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredSeventySevenSummerhillRoadMember	[2]
Office [Member] | 343 Thornall Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1991
Acquired	2006
Initial Costs, Land	6,027,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFortyThreeThornallStreetMember
Initial Costs, Building and Improvements	39,101,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFortyThreeThornallStreetMember
Costs Capitalized Subsequent to Acquisition	3,302,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFortyThreeThornallStreetMember
Gross Amount at Which Carried at Close of Period, Land	6,027,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFortyThreeThornallStreetMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	42,403,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFortyThreeThornallStreetMember
Total	48,430,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFortyThreeThornallStreetMember
Accumulated depreciation	10,907,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFortyThreeThornallStreetMember	[2]
Office [Member] | 500 College Road East [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	1998
Initial Costs, Land	614,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredCollegeRoadEastMember
Initial Costs, Building and Improvements	20,626,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredCollegeRoadEastMember
Costs Capitalized Subsequent to Acquisition	6,178,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredCollegeRoadEastMember
Gross Amount at Which Carried at Close of Period, Land	614,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredCollegeRoadEastMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	26,804,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredCollegeRoadEastMember
Total	27,418,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredCollegeRoadEastMember
Accumulated depreciation	11,800,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredCollegeRoadEastMember	[2]
Office [Member] | 581 Main Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1991
Acquired	1997
Initial Costs, Land	3,237,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredEightyOneMainStreetMember
Initial Costs, Building and Improvements	12,949,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredEightyOneMainStreetMember
Costs Capitalized Subsequent to Acquisition	26,205,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredEightyOneMainStreetMember
Gross Amount at Which Carried at Close of Period, Land	8,115,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredEightyOneMainStreetMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	34,276,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredEightyOneMainStreetMember
Total	42,391,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredEightyOneMainStreetMember
Accumulated depreciation	15,447,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredEightyOneMainStreetMember	[2]
Office [Member] | 2 Paragon Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	2005
Initial Costs, Land	999,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoParagonWayMember
Initial Costs, Building and Improvements	4,619,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoParagonWayMember
Costs Capitalized Subsequent to Acquisition	876,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoParagonWayMember
Gross Amount at Which Carried at Close of Period, Land	999,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoParagonWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,495,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoParagonWayMember
Total	6,494,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoParagonWayMember
Accumulated depreciation	1,567,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoParagonWayMember	[2]
Office [Member] | 3 Paragon Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1991
Acquired	2005
Initial Costs, Land	1,423,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeParagonWayMember
Initial Costs, Building and Improvements	6,041,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeParagonWayMember
Costs Capitalized Subsequent to Acquisition	1,550,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeParagonWayMember
Gross Amount at Which Carried at Close of Period, Land	1,423,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeParagonWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,591,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeParagonWayMember
Total	9,014,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeParagonWayMember
Accumulated depreciation	2,235,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeParagonWayMember	[2]
Office [Member] | 4 Paragon Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2002
Acquired	2005
Initial Costs, Land	1,961,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourParagonWayMember
Initial Costs, Building and Improvements	8,827,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourParagonWayMember
Costs Capitalized Subsequent to Acquisition	(512,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourParagonWayMember
Gross Amount at Which Carried at Close of Period, Land	1,961,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourParagonWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,315,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourParagonWayMember
Total	10,276,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourParagonWayMember
Accumulated depreciation	1,958,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourParagonWayMember	[2]
Office [Member] | 100 Willowbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	2005
Initial Costs, Land	1,264,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWillowbrookRoadMember
Initial Costs, Building and Improvements	5,573,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWillowbrookRoadMember
Costs Capitalized Subsequent to Acquisition	804,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWillowbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	1,264,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWillowbrookRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	6,377,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWillowbrookRoadMember
Total	7,641,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWillowbrookRoadMember
Accumulated depreciation	1,844,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWillowbrookRoadMember	[2]
Office [Member] | 23 Main Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1977
Acquired	2005
Related Encumbrances	29,210,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyThreeMainStreetMember
Initial Costs, Land	4,336,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyThreeMainStreetMember
Initial Costs, Building and Improvements	19,544,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyThreeMainStreetMember
Costs Capitalized Subsequent to Acquisition	9,133,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyThreeMainStreetMember
Gross Amount at Which Carried at Close of Period, Land	4,336,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyThreeMainStreetMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	28,677,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyThreeMainStreetMember
Total	33,013,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyThreeMainStreetMember
Accumulated depreciation	11,514,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyThreeMainStreetMember	[2]
Office [Member] | One River Center, Building 1 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	2004
Related Encumbrances	10,879,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingOneMember
Initial Costs, Land	3,070,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingOneMember
Initial Costs, Building and Improvements	17,414,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingOneMember
Costs Capitalized Subsequent to Acquisition	3,881,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingOneMember
Gross Amount at Which Carried at Close of Period, Land	2,451,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingOneMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	21,914,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingOneMember
Total	24,365,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingOneMember
Accumulated depreciation	7,051,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingOneMember	[2]
Office [Member] | One River Center, Building 2 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	2004
Related Encumbrances	12,204,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingTwoMember
Initial Costs, Land	2,468,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingTwoMember
Initial Costs, Building and Improvements	15,043,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingTwoMember
Costs Capitalized Subsequent to Acquisition	3,702,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingTwoMember
Gross Amount at Which Carried at Close of Period, Land	2,452,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingTwoMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	18,761,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingTwoMember
Total	21,213,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingTwoMember
Accumulated depreciation	4,628,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingTwoMember	[2]
Office [Member] | One River Center, Building 3 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	2004
Related Encumbrances	19,393,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingThreeMember
Initial Costs, Land	4,051,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingThreeMember
Initial Costs, Building and Improvements	24,790,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingThreeMember
Costs Capitalized Subsequent to Acquisition	6,030,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingThreeMember
Gross Amount at Which Carried at Close of Period, Land	4,627,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingThreeMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	30,244,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingThreeMember
Total	34,871,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingThreeMember
Accumulated depreciation	8,503,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneRiverCenterBuildingThreeMember	[2]
Office [Member] | 3600 Route 66 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1995
Initial Costs, Land	1,098,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeThousandSixHundredRouteSixtySixMember
Initial Costs, Building and Improvements	18,146,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeThousandSixHundredRouteSixtySixMember
Costs Capitalized Subsequent to Acquisition	10,455,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeThousandSixHundredRouteSixtySixMember
Gross Amount at Which Carried at Close of Period, Land	1,098,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeThousandSixHundredRouteSixtySixMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	28,601,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeThousandSixHundredRouteSixtySixMember
Total	29,699,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeThousandSixHundredRouteSixtySixMember
Accumulated depreciation	10,121,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeThousandSixHundredRouteSixtySixMember	[2]
Office [Member] | 1305 Campus Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1995
Initial Costs, Land	335,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiveCampusParkwayMember
Initial Costs, Building and Improvements	2,560,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiveCampusParkwayMember
Costs Capitalized Subsequent to Acquisition	648,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiveCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Land	291,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiveCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,252,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiveCampusParkwayMember
Total	3,543,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiveCampusParkwayMember
Accumulated depreciation	1,463,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiveCampusParkwayMember	[2]
Office [Member] | 1350 Campus Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1995
Initial Costs, Land	454,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiftyCampusParkwayMember
Initial Costs, Building and Improvements	7,134,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiftyCampusParkwayMember
Costs Capitalized Subsequent to Acquisition	823,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiftyCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Land	454,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiftyCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,957,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiftyCampusParkwayMember
Total	8,411,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiftyCampusParkwayMember
Accumulated depreciation	3,805,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFiftyCampusParkwayMember	[2]
Office [Member] | 325 Columbia Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1994
Initial Costs, Land	1,564,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTwentyFiveColumbiaParkwayMember
Costs Capitalized Subsequent to Acquisition	17,390,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTwentyFiveColumbiaParkwayMember
Gross Amount at Which Carried at Close of Period, Land	1,564,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTwentyFiveColumbiaParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	17,390,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTwentyFiveColumbiaParkwayMember
Total	18,954,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTwentyFiveColumbiaParkwayMember
Accumulated depreciation	10,299,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredTwentyFiveColumbiaParkwayMember	[2]
Office [Member] | 250 Johnson Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1977
Acquired	1997
Initial Costs, Land	2,004,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyJohnsonRoadMember
Initial Costs, Building and Improvements	8,016,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyJohnsonRoadMember
Costs Capitalized Subsequent to Acquisition	(4,825,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyJohnsonRoadMember
Gross Amount at Which Carried at Close of Period, Land	930,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyJohnsonRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,265,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyJohnsonRoadMember
Total	5,195,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyJohnsonRoadMember
Accumulated depreciation	1,618,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyJohnsonRoadMember	[2]
Office [Member] | 201 Littleton Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1979
Acquired	1997
Initial Costs, Land	2,407,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneLittletonRoadMember
Initial Costs, Building and Improvements	9,627,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneLittletonRoadMember
Costs Capitalized Subsequent to Acquisition	3,505,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneLittletonRoadMember
Gross Amount at Which Carried at Close of Period, Land	2,407,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneLittletonRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	13,132,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneLittletonRoadMember
Total	15,539,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneLittletonRoadMember
Accumulated depreciation	5,265,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneLittletonRoadMember	[2]
Office [Member] | 4 Campus Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	2001
Initial Costs, Land	5,213,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCampusDriveMember
Initial Costs, Building and Improvements	20,984,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCampusDriveMember
Costs Capitalized Subsequent to Acquisition	3,389,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCampusDriveMember
Gross Amount at Which Carried at Close of Period, Land	5,213,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCampusDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	24,373,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCampusDriveMember
Total	29,586,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCampusDriveMember
Accumulated depreciation	8,918,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCampusDriveMember	[2]
Office [Member] | 6 Campus Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	2001
Initial Costs, Land	4,411,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCampusDriveMember
Initial Costs, Building and Improvements	17,796,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCampusDriveMember
Costs Capitalized Subsequent to Acquisition	3,287,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCampusDriveMember
Gross Amount at Which Carried at Close of Period, Land	4,411,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCampusDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	21,083,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCampusDriveMember
Total	25,494,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCampusDriveMember
Accumulated depreciation	7,867,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCampusDriveMember	[2]
Office [Member] | 7 Campus Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1982
Acquired	1998
Initial Costs, Land	1,932,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenCampusDriveMember
Initial Costs, Building and Improvements	27,788,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenCampusDriveMember
Costs Capitalized Subsequent to Acquisition	6,310,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenCampusDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,932,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenCampusDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	34,098,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenCampusDriveMember
Total	36,030,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenCampusDriveMember
Accumulated depreciation	14,566,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenCampusDriveMember	[2]
Office [Member] | 8 Campus Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1998
Initial Costs, Land	1,865,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightCampusDriveMember
Initial Costs, Building and Improvements	35,456,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightCampusDriveMember
Costs Capitalized Subsequent to Acquisition	4,789,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightCampusDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,865,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightCampusDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	40,245,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightCampusDriveMember
Total	42,110,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightCampusDriveMember
Accumulated depreciation	15,971,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightCampusDriveMember	[2]
Office [Member] | 9 Campus Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	2001
Initial Costs, Land	3,277,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineCampusDriveMember
Initial Costs, Building and Improvements	11,796,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineCampusDriveMember
Costs Capitalized Subsequent to Acquisition	16,318,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineCampusDriveMember
Gross Amount at Which Carried at Close of Period, Land	5,842,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineCampusDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	25,549,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineCampusDriveMember
Total	31,391,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineCampusDriveMember
Accumulated depreciation	9,647,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineCampusDriveMember	[2]
Office [Member] | 4 Century Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	2004
Initial Costs, Land	1,787,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCenturyDriveMember
Initial Costs, Building and Improvements	9,575,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCenturyDriveMember
Costs Capitalized Subsequent to Acquisition	1,862,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCenturyDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,787,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCenturyDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	11,437,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCenturyDriveMember
Total	13,224,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCenturyDriveMember
Accumulated depreciation	3,461,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourCenturyDriveMember	[2]
Office [Member] | 5 Century Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	2004
Initial Costs, Land	1,762,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveCenturyDriveMember
Initial Costs, Building and Improvements	9,341,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveCenturyDriveMember
Costs Capitalized Subsequent to Acquisition	2,390,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveCenturyDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,762,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveCenturyDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	11,731,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveCenturyDriveMember
Total	13,493,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveCenturyDriveMember
Accumulated depreciation	3,616,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveCenturyDriveMember	[2]
Office [Member] | 6 Century Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	2004
Initial Costs, Land	1,289,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCenturyDriveMember
Initial Costs, Building and Improvements	6,848,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCenturyDriveMember
Costs Capitalized Subsequent to Acquisition	2,178,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCenturyDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,289,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCenturyDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	9,026,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCenturyDriveMember
Total	10,315,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCenturyDriveMember
Accumulated depreciation	2,855,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCenturyDriveMember	[2]
Office [Member] | 2 Dryden Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1998
Initial Costs, Land	778,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoDrydenWayMember
Initial Costs, Building and Improvements	420,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoDrydenWayMember
Costs Capitalized Subsequent to Acquisition	110,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoDrydenWayMember
Gross Amount at Which Carried at Close of Period, Land	778,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoDrydenWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	530,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoDrydenWayMember
Total	1,308,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoDrydenWayMember
Accumulated depreciation	245,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoDrydenWayMember	[2]
Office [Member] | 4 Gatehall Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	2000
Initial Costs, Land	8,452,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourGatehallDriveMember
Initial Costs, Building and Improvements	33,929,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourGatehallDriveMember
Costs Capitalized Subsequent to Acquisition	4,433,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourGatehallDriveMember
Gross Amount at Which Carried at Close of Period, Land	8,452,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourGatehallDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	38,362,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourGatehallDriveMember
Total	46,814,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourGatehallDriveMember
Accumulated depreciation	14,754,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourGatehallDriveMember	[2]
Office [Member] | 2 Hilton Court [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1991
Acquired	1998
Initial Costs, Land	1,971,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHiltonCourtMember
Initial Costs, Building and Improvements	32,007,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHiltonCourtMember
Costs Capitalized Subsequent to Acquisition	4,958,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHiltonCourtMember
Gross Amount at Which Carried at Close of Period, Land	1,971,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHiltonCourtMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	36,965,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHiltonCourtMember
Total	38,936,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHiltonCourtMember
Accumulated depreciation	16,047,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHiltonCourtMember	[2]
Office [Member] | 1633 Littleton Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1978
Acquired	2002
Initial Costs, Land	2,283,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandSixHundredThirtyThreeLittletonRoadMember
Initial Costs, Building and Improvements	9,550,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandSixHundredThirtyThreeLittletonRoadMember
Costs Capitalized Subsequent to Acquisition	163,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandSixHundredThirtyThreeLittletonRoadMember
Gross Amount at Which Carried at Close of Period, Land	2,355,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandSixHundredThirtyThreeLittletonRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	9,641,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandSixHundredThirtyThreeLittletonRoadMember
Total	11,996,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandSixHundredThirtyThreeLittletonRoadMember
Accumulated depreciation	4,227,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandSixHundredThirtyThreeLittletonRoadMember	[2]
Office [Member] | 600 Parsippany Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1978
Acquired	1994
Initial Costs, Land	1,257,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredParsippanyRoadMember
Initial Costs, Building and Improvements	5,594,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredParsippanyRoadMember
Costs Capitalized Subsequent to Acquisition	3,277,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredParsippanyRoadMember
Gross Amount at Which Carried at Close of Period, Land	1,257,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredParsippanyRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,871,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredParsippanyRoadMember
Total	10,128,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredParsippanyRoadMember
Accumulated depreciation	4,515,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredParsippanyRoadMember	[2]
Office [Member] | 1 Sylvan Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1998
Initial Costs, Land	1,689,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSylvanWayMember
Initial Costs, Building and Improvements	24,699,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSylvanWayMember
Costs Capitalized Subsequent to Acquisition	2,826,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSylvanWayMember
Gross Amount at Which Carried at Close of Period, Land	1,021,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSylvanWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	28,193,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSylvanWayMember
Total	29,214,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSylvanWayMember
Accumulated depreciation	12,489,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSylvanWayMember	[2]
Office [Member] | 4 Sylvan Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	2009
Related Encumbrances	14,575,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSylvanWayMember
Initial Costs, Land	2,400,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSylvanWayMember
Initial Costs, Building and Improvements	13,486,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSylvanWayMember
Costs Capitalized Subsequent to Acquisition	(8,399,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSylvanWayMember
Gross Amount at Which Carried at Close of Period, Land	1,131,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSylvanWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	6,356,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSylvanWayMember
Total	7,487,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSylvanWayMember
Accumulated depreciation	1,149,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSylvanWayMember	[2]
Office [Member] | 5 Sylvan Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1998
Initial Costs, Land	1,160,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSylvanWayMember
Initial Costs, Building and Improvements	25,214,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSylvanWayMember
Costs Capitalized Subsequent to Acquisition	2,723,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSylvanWayMember
Gross Amount at Which Carried at Close of Period, Land	1,161,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSylvanWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	27,936,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSylvanWayMember
Total	29,097,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSylvanWayMember
Accumulated depreciation	11,307,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSylvanWayMember	[2]
Office [Member] | 7 Sylvan Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1998
Initial Costs, Land	2,084,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSylvanWayMember
Initial Costs, Building and Improvements	26,083,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSylvanWayMember
Costs Capitalized Subsequent to Acquisition	841,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSylvanWayMember
Gross Amount at Which Carried at Close of Period, Land	2,084,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSylvanWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	26,924,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSylvanWayMember
Total	29,008,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSylvanWayMember
Accumulated depreciation	11,063,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSylvanWayMember	[2]
Office [Member] | 14 Sylvan Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2013
Acquired	2013
Initial Costs, Land	13,049,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenSylvanWayMember
Initial Costs, Building and Improvements	26,841,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenSylvanWayMember
Costs Capitalized Subsequent to Acquisition	8,540,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenSylvanWayMember
Gross Amount at Which Carried at Close of Period, Land	13,049,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenSylvanWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	35,381,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenSylvanWayMember
Total	48,430,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenSylvanWayMember
Accumulated depreciation	1,841,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenSylvanWayMember	[2]
Office [Member] | 20 Waterview Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	2009
Related Encumbrances	24,678,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyWaterviewBoulevardMember
Initial Costs, Land	4,500,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyWaterviewBoulevardMember
Initial Costs, Building and Improvements	27,246,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyWaterviewBoulevardMember
Costs Capitalized Subsequent to Acquisition	(1,233,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyWaterviewBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	4,208,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyWaterviewBoulevardMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	26,305,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyWaterviewBoulevardMember
Total	30,513,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyWaterviewBoulevardMember
Accumulated depreciation	4,736,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyWaterviewBoulevardMember	[2]
Office [Member] | 35 Waterview Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	2006
Initial Costs, Land	5,133,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyFiveWaterviewBoulevardMember
Initial Costs, Building and Improvements	28,059,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyFiveWaterviewBoulevardMember
Costs Capitalized Subsequent to Acquisition	887,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyFiveWaterviewBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	5,133,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyFiveWaterviewBoulevardMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	28,946,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyFiveWaterviewBoulevardMember
Total	34,079,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyFiveWaterviewBoulevardMember
Accumulated depreciation	7,304,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyFiveWaterviewBoulevardMember	[2]
Office [Member] | 5 Wood Hollow Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1979
Acquired	2004
Initial Costs, Land	5,302,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWoodHollowRoadMember
Initial Costs, Building and Improvements	26,488,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWoodHollowRoadMember
Costs Capitalized Subsequent to Acquisition	16,441,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWoodHollowRoadMember
Gross Amount at Which Carried at Close of Period, Land	5,302,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWoodHollowRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	42,929,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWoodHollowRoadMember
Total	48,231,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWoodHollowRoadMember
Accumulated depreciation	15,710,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWoodHollowRoadMember	[2]
Office [Member] | 999 Riverview Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1995
Initial Costs, Land	476,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredNinetyNineRiverviewDriveMember
Initial Costs, Building and Improvements	6,024,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredNinetyNineRiverviewDriveMember
Costs Capitalized Subsequent to Acquisition	2,793,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredNinetyNineRiverviewDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,102,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredNinetyNineRiverviewDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,191,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredNinetyNineRiverviewDriveMember
Total	9,293,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredNinetyNineRiverviewDriveMember
Accumulated depreciation	4,018,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredNinetyNineRiverviewDriveMember	[2]
Office [Member] | 222 Mt. Airy Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1986
Acquired	1996
Initial Costs, Land	775,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyTwoMountAiryRoadMember
Initial Costs, Building and Improvements	3,636,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyTwoMountAiryRoadMember
Costs Capitalized Subsequent to Acquisition	2,278,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyTwoMountAiryRoadMember
Gross Amount at Which Carried at Close of Period, Land	775,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyTwoMountAiryRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,914,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyTwoMountAiryRoadMember
Total	6,689,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyTwoMountAiryRoadMember
Accumulated depreciation	2,662,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyTwoMountAiryRoadMember	[2]
Office [Member] | 233 Mt. Airy Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1996
Initial Costs, Land	1,034,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyThreeMountAiryRoadMember
Initial Costs, Building and Improvements	5,033,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyThreeMountAiryRoadMember
Costs Capitalized Subsequent to Acquisition	2,204,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyThreeMountAiryRoadMember
Gross Amount at Which Carried at Close of Period, Land	1,034,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyThreeMountAiryRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,237,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyThreeMountAiryRoadMember
Total	8,271,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyThreeMountAiryRoadMember
Accumulated depreciation	2,737,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyThreeMountAiryRoadMember	[2]
Office [Member] | 440 Route 22 East [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	2010
Initial Costs, Land	3,986,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredFortyRouteTwentyTwoEastMember
Initial Costs, Building and Improvements	13,658,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredFortyRouteTwentyTwoEastMember
Costs Capitalized Subsequent to Acquisition	4,814,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredFortyRouteTwentyTwoEastMember
Gross Amount at Which Carried at Close of Period, Land	3,986,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredFortyRouteTwentyTwoEastMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	18,472,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredFortyRouteTwentyTwoEastMember
Total	22,458,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredFortyRouteTwentyTwoEastMember
Accumulated depreciation	3,661,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredFortyRouteTwentyTwoEastMember	[2]
Office [Member] | 721 Route 202/206 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1997
Initial Costs, Land	6,730,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredTwentyOneRouteTwoHundredTwoSlashTwoHundredSixMember
Initial Costs, Building and Improvements	26,919,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredTwentyOneRouteTwoHundredTwoSlashTwoHundredSixMember
Costs Capitalized Subsequent to Acquisition	9,711,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredTwentyOneRouteTwoHundredTwoSlashTwoHundredSixMember
Gross Amount at Which Carried at Close of Period, Land	6,730,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredTwentyOneRouteTwoHundredTwoSlashTwoHundredSixMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	36,630,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredTwentyOneRouteTwoHundredTwoSlashTwoHundredSixMember
Total	43,360,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredTwentyOneRouteTwoHundredTwoSlashTwoHundredSixMember
Accumulated depreciation	18,700,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredTwentyOneRouteTwoHundredTwoSlashTwoHundredSixMember	[2]
Office [Member] | 10 Independence [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	2009
Related Encumbrances	16,924,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenIndependenceMember
Initial Costs, Land	2,300,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenIndependenceMember
Initial Costs, Building and Improvements	15,499,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenIndependenceMember
Costs Capitalized Subsequent to Acquisition	(6,031,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenIndependenceMember
Gross Amount at Which Carried at Close of Period, Land	1,482,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenIndependenceMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	10,286,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenIndependenceMember
Total	11,768,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenIndependenceMember
Accumulated depreciation	1,573,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenIndependenceMember	[2]
Office [Member] | 100 Walnut Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	1994
Related Encumbrances	18,542,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWalnutAvenueMember
Costs Capitalized Subsequent to Acquisition	17,640,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWalnutAvenueMember
Gross Amount at Which Carried at Close of Period, Land	1,822,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWalnutAvenueMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	15,818,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWalnutAvenueMember
Total	17,640,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWalnutAvenueMember
Accumulated depreciation	10,340,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredWalnutAvenueMember	[2]
Office [Member] | 6 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1973
Acquired	1994
Initial Costs, Land	250,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	1,784,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	250,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,784,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCommerceDriveMember
Total	2,034,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCommerceDriveMember
Accumulated depreciation	712,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixCommerceDriveMember	[2]
Office [Member] | 11 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1981
Acquired	1994
Initial Costs, Land	470,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	5,556,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	470,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,556,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceDriveMember
Total	6,026,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceDriveMember
Accumulated depreciation	4,655,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceDriveMember	[2]
Office [Member] | 12 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1967
Acquired	1997
Initial Costs, Land	887,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveCommerceDriveMember
Initial Costs, Building and Improvements	3,549,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	1,543,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	887,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,092,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveCommerceDriveMember
Total	5,979,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveCommerceDriveMember
Accumulated depreciation	2,354,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveCommerceDriveMember	[2]
Office [Member] | 14 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1971
Acquired	2003
Initial Costs, Land	1,283,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenCommerceDriveMember
Initial Costs, Building and Improvements	6,344,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	1,043,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,283,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,387,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenCommerceDriveMember
Total	8,670,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenCommerceDriveMember
Accumulated depreciation	2,185,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourteenCommerceDriveMember	[2]
Office [Member] | 20 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1994
Initial Costs, Land	2,346,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	21,447,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	2,346,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	21,447,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceDriveMember
Total	23,793,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceDriveMember
Accumulated depreciation	11,154,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceDriveMember	[2]
Office [Member] | 25 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1971
Acquired	2002
Initial Costs, Land	1,520,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyFiveCommerceDriveMember
Initial Costs, Building and Improvements	6,186,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyFiveCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	1,028,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyFiveCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,520,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyFiveCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,214,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyFiveCommerceDriveMember
Total	8,734,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyFiveCommerceDriveMember
Accumulated depreciation	3,132,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyFiveCommerceDriveMember	[2]
Office [Member] | 65 Jackson Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	1994
Initial Costs, Land	541,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyFiveJacksonDriveMember
Costs Capitalized Subsequent to Acquisition	6,223,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyFiveJacksonDriveMember
Gross Amount at Which Carried at Close of Period, Land	542,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyFiveJacksonDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	6,222,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyFiveJacksonDriveMember
Total	6,764,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyFiveJacksonDriveMember
Accumulated depreciation	4,100,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyFiveJacksonDriveMember	[2]
Office [Member] | 890 Mountain Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1977
Acquired	1997
Initial Costs, Land	2,796,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredNinetyMountainRoadMember
Initial Costs, Building and Improvements	11,185,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredNinetyMountainRoadMember
Costs Capitalized Subsequent to Acquisition	6,167,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredNinetyMountainRoadMember
Gross Amount at Which Carried at Close of Period, Land	3,765,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredNinetyMountainRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	16,383,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredNinetyMountainRoadMember
Total	20,148,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredNinetyMountainRoadMember
Accumulated depreciation	6,843,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredNinetyMountainRoadMember	[2]
Office [Member] | 125 Broad Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1970
Acquired	2007
Initial Costs, Land	50,191,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveBroadStreetMember
Initial Costs, Building and Improvements	207,002,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveBroadStreetMember
Costs Capitalized Subsequent to Acquisition	48,289,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveBroadStreetMember
Gross Amount at Which Carried at Close of Period, Land	50,191,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveBroadStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	255,291,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveBroadStreetMember
Total	305,482,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveBroadStreetMember
Accumulated depreciation	48,103,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveBroadStreetMember	[2]
Office [Member] | 100 Clearbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1975
Acquired	1997
Initial Costs, Land	220,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredClearbrookRoadMember
Initial Costs, Building and Improvements	5,366,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredClearbrookRoadMember
Costs Capitalized Subsequent to Acquisition	1,653,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredClearbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	220,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredClearbrookRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,019,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredClearbrookRoadMember
Total	7,239,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredClearbrookRoadMember
Accumulated depreciation	3,117,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredClearbrookRoadMember	[2]
Office [Member] | 101 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1971
Acquired	1997
Initial Costs, Land	267,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveBoulevardMember
Initial Costs, Building and Improvements	5,838,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	(3,536,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	101,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,468,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveBoulevardMember
Total	2,569,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveBoulevardMember
Accumulated depreciation	1,060,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveBoulevardMember	[2]
Office [Member] | 1 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1980
Acquired	1997
Initial Costs, Land	66,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSkylineDriveMember
Initial Costs, Building and Improvements	1,711,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	210,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	66,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,921,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSkylineDriveMember
Total	1,987,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSkylineDriveMember
Accumulated depreciation	890,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneSkylineDriveMember
Office [Member] | 2 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1997
Initial Costs, Land	109,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSkylineDriveMember
Initial Costs, Building and Improvements	3,128,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	1,474,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	109,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,602,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSkylineDriveMember
Total	4,711,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSkylineDriveMember
Accumulated depreciation	2,057,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoSkylineDriveMember
Office [Member] | 7 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1998
Initial Costs, Land	330,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSkylineDriveMember
Initial Costs, Building and Improvements	13,013,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	3,172,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	330,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	16,185,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSkylineDriveMember
Total	16,515,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSkylineDriveMember
Accumulated depreciation	6,868,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenSkylineDriveMember
Office [Member] | 17 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1997
Initial Costs, Building and Improvements	7,269,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventeenSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	1,484,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventeenSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,753,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventeenSkylineDriveMember
Total	8,753,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventeenSkylineDriveMember
Accumulated depreciation	3,725,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventeenSkylineDriveMember
Office [Member] | 1 Barker Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1975
Acquired	1997
Initial Costs, Land	208,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBarkerAvenueMember
Initial Costs, Building and Improvements	9,629,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBarkerAvenueMember
Costs Capitalized Subsequent to Acquisition	2,546,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBarkerAvenueMember
Gross Amount at Which Carried at Close of Period, Land	207,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBarkerAvenueMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	12,176,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBarkerAvenueMember
Total	12,383,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBarkerAvenueMember
Accumulated depreciation	5,083,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneBarkerAvenueMember
Office [Member] | 3 Barker Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	1997
Initial Costs, Land	122,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeBarkerAvenueMember
Initial Costs, Building and Improvements	7,864,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeBarkerAvenueMember
Costs Capitalized Subsequent to Acquisition	1,781,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeBarkerAvenueMember
Gross Amount at Which Carried at Close of Period, Land	122,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeBarkerAvenueMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	9,645,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeBarkerAvenueMember
Total	9,767,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeBarkerAvenueMember
Accumulated depreciation	4,128,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeBarkerAvenueMember
Office [Member] | 50 Main Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	1997
Initial Costs, Land	564,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyMainStreetMember
Initial Costs, Building and Improvements	48,105,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyMainStreetMember
Costs Capitalized Subsequent to Acquisition	14,371,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyMainStreetMember
Gross Amount at Which Carried at Close of Period, Land	564,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyMainStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	62,476,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyMainStreetMember
Total	63,040,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyMainStreetMember
Accumulated depreciation	27,557,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyMainStreetMember
Office [Member] | 11 Martine Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1997
Initial Costs, Land	127,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenMartineAvenueMember
Initial Costs, Building and Improvements	26,833,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenMartineAvenueMember
Costs Capitalized Subsequent to Acquisition	9,209,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenMartineAvenueMember
Gross Amount at Which Carried at Close of Period, Land	127,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenMartineAvenueMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	36,042,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenMartineAvenueMember
Total	36,169,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenMartineAvenueMember
Accumulated depreciation	16,385,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenMartineAvenueMember
Office [Member] | 1 Water Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1979
Acquired	1997
Initial Costs, Land	211,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWaterStreetMember
Initial Costs, Building and Improvements	5,382,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWaterStreetMember
Costs Capitalized Subsequent to Acquisition	1,101,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWaterStreetMember
Gross Amount at Which Carried at Close of Period, Land	211,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWaterStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	6,483,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWaterStreetMember
Total	6,694,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWaterStreetMember
Accumulated depreciation	2,835,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWaterStreetMember
Office [Member] | 1 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1982
Acquired	1997
Initial Costs, Land	1,104,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveBoulevardMember
Initial Costs, Building and Improvements	11,904,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	3,281,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	1,105,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	15,184,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveBoulevardMember
Total	16,289,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveBoulevardMember
Accumulated depreciation	6,592,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveBoulevardMember
Office [Member] | 3 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1997
Initial Costs, Land	385,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeExecutiveBoulevardMember
Initial Costs, Building and Improvements	6,256,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	2,103,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	385,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,359,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeExecutiveBoulevardMember
Total	8,744,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeExecutiveBoulevardMember
Accumulated depreciation	3,657,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeExecutiveBoulevardMember
Office [Member] | 3 Odell Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	2003
Initial Costs, Land	1,322,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeOdellPlazaMember
Initial Costs, Building and Improvements	4,777,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeOdellPlazaMember
Costs Capitalized Subsequent to Acquisition	2,321,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeOdellPlazaMember
Gross Amount at Which Carried at Close of Period, Land	1,322,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeOdellPlazaMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,098,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeOdellPlazaMember
Total	8,420,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeOdellPlazaMember
Accumulated depreciation	3,019,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeOdellPlazaMember
Office [Member] | 1201 Connecticut Avenue, NW [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1940
Acquired	1999
Initial Costs, Land	14,228,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredOneConnecticutAvenueNWMember
Initial Costs, Building and Improvements	18,571,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredOneConnecticutAvenueNWMember
Costs Capitalized Subsequent to Acquisition	5,846,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredOneConnecticutAvenueNWMember
Gross Amount at Which Carried at Close of Period, Land	14,228,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredOneConnecticutAvenueNWMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	24,417,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredOneConnecticutAvenueNWMember
Total	38,645,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredOneConnecticutAvenueNWMember
Accumulated depreciation	10,322,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredOneConnecticutAvenueNWMember
Office [Member] | 1400 L Street, NW [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1998
Initial Costs, Land	13,054,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredLStreetNWMember
Initial Costs, Building and Improvements	27,423,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredLStreetNWMember
Costs Capitalized Subsequent to Acquisition	7,909,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredLStreetNWMember
Gross Amount at Which Carried at Close of Period, Land	13,054,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredLStreetNWMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	35,332,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredLStreetNWMember
Total	48,386,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredLStreetNWMember
Accumulated depreciation	17,571,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredLStreetNWMember
Office [Member] | 9200 Edmonston Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Acquired	2006
Related Encumbrances	3,951,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineThousandTwoHundredEdmonstonRoadMember
Initial Costs, Land	1,547,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineThousandTwoHundredEdmonstonRoadMember
Initial Costs, Building and Improvements	4,131,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineThousandTwoHundredEdmonstonRoadMember
Costs Capitalized Subsequent to Acquisition	(2,484,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineThousandTwoHundredEdmonstonRoadMember
Gross Amount at Which Carried at Close of Period, Land	610,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineThousandTwoHundredEdmonstonRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,584,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineThousandTwoHundredEdmonstonRoadMember
Total	3,194,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineThousandTwoHundredEdmonstonRoadMember
Accumulated depreciation	1,336,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineThousandTwoHundredEdmonstonRoadMember
Office [Member] | 6301 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Acquired	2006
Initial Costs, Land	5,168,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredOneIvyLaneMember
Initial Costs, Building and Improvements	14,706,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredOneIvyLaneMember
Costs Capitalized Subsequent to Acquisition	1,289,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredOneIvyLaneMember
Gross Amount at Which Carried at Close of Period, Land	5,168,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredOneIvyLaneMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	15,995,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredOneIvyLaneMember
Total	21,163,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredOneIvyLaneMember
Accumulated depreciation	3,981,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredOneIvyLaneMember
Office [Member] | 6303 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Acquired	2006
Initial Costs, Land	5,115,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredThreeIvyLaneMember
Initial Costs, Building and Improvements	13,860,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredThreeIvyLaneMember
Costs Capitalized Subsequent to Acquisition	770,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredThreeIvyLaneMember
Gross Amount at Which Carried at Close of Period, Land	5,115,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredThreeIvyLaneMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	14,630,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredThreeIvyLaneMember
Total	19,745,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredThreeIvyLaneMember
Accumulated depreciation	4,236,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredThreeIvyLaneMember
Office [Member] | 6305 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Acquired	2006
Initial Costs, Land	5,615,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredFiveIvyLaneMember
Initial Costs, Building and Improvements	14,420,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredFiveIvyLaneMember
Costs Capitalized Subsequent to Acquisition	1,071,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredFiveIvyLaneMember
Gross Amount at Which Carried at Close of Period, Land	5,615,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredFiveIvyLaneMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	15,491,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredFiveIvyLaneMember
Total	21,106,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredFiveIvyLaneMember
Accumulated depreciation	3,835,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandThreeHundredFiveIvyLaneMember
Office [Member] | 6404 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	2006
Initial Costs, Land	7,578,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredFourIvyLaneMember
Initial Costs, Building and Improvements	20,785,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredFourIvyLaneMember
Costs Capitalized Subsequent to Acquisition	1,998,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredFourIvyLaneMember
Gross Amount at Which Carried at Close of Period, Land	7,578,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredFourIvyLaneMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	22,783,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredFourIvyLaneMember
Total	30,361,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredFourIvyLaneMember
Accumulated depreciation	6,684,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredFourIvyLaneMember
Office [Member] | 6406 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1991
Acquired	2006
Initial Costs, Land	7,514,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredSixIvyLaneMember
Initial Costs, Building and Improvements	21,152,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredSixIvyLaneMember
Costs Capitalized Subsequent to Acquisition	5,770,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredSixIvyLaneMember
Gross Amount at Which Carried at Close of Period, Land	7,514,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredSixIvyLaneMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	26,922,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredSixIvyLaneMember
Total	34,436,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredSixIvyLaneMember
Accumulated depreciation	5,119,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredSixIvyLaneMember
Office [Member] | 6411 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Acquired	2006
Initial Costs, Land	6,867,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredElevenIvyLaneMember
Initial Costs, Building and Improvements	17,470,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredElevenIvyLaneMember
Costs Capitalized Subsequent to Acquisition	965,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredElevenIvyLaneMember
Gross Amount at Which Carried at Close of Period, Land	6,867,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredElevenIvyLaneMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	18,435,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredElevenIvyLaneMember
Total	25,302,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredElevenIvyLaneMember
Accumulated depreciation	4,789,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixThousandFourHundredElevenIvyLaneMember
Office [Member] | 4200 Parliament Place [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1998
Initial Costs, Land	2,114,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourThousandTwoHundredParliamentPlaceMember
Initial Costs, Building and Improvements	13,546,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourThousandTwoHundredParliamentPlaceMember
Costs Capitalized Subsequent to Acquisition	1,227,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourThousandTwoHundredParliamentPlaceMember
Gross Amount at Which Carried at Close of Period, Land	1,393,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourThousandTwoHundredParliamentPlaceMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	15,494,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourThousandTwoHundredParliamentPlaceMember
Total	16,887,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourThousandTwoHundredParliamentPlaceMember
Accumulated depreciation	7,227,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OfficeBuildingMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourThousandTwoHundredParliamentPlaceMember
Office And Office/Flex Buildings [Member] | 3 Terri Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1991
Acquired	1998
Initial Costs, Land	652,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeTerriLaneMember
Initial Costs, Building and Improvements	3,433,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeTerriLaneMember
Costs Capitalized Subsequent to Acquisition	1,537,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeTerriLaneMember
Gross Amount at Which Carried at Close of Period, Land	658,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeTerriLaneMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,964,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeTerriLaneMember	[1]
Total	5,622,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeTerriLaneMember
Accumulated depreciation	2,110,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeTerriLaneMember	[2]
Office And Office/Flex Buildings [Member] | 5 Terri Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1992
Acquired	1998
Initial Costs, Land	564,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveTerriLaneMember
Initial Costs, Building and Improvements	3,792,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveTerriLaneMember
Costs Capitalized Subsequent to Acquisition	2,444,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveTerriLaneMember
Gross Amount at Which Carried at Close of Period, Land	569,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveTerriLaneMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	6,231,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveTerriLaneMember	[1]
Total	6,800,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveTerriLaneMember
Accumulated depreciation	2,784,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveTerriLaneMember	[2]
Office And Office/Flex Buildings [Member] | 2 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1986
Acquired	1999
Initial Costs, Land	723,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCommerceDriveMember
Initial Costs, Building and Improvements	2,893,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	544,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	723,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,437,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCommerceDriveMember	[1]
Total	4,160,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCommerceDriveMember
Accumulated depreciation	1,351,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCommerceDriveMember	[2]
Office And Office/Flex Buildings [Member] | 101 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1998
Initial Costs, Land	422,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneCommerceDriveMember
Initial Costs, Building and Improvements	3,528,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	436,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	426,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,960,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneCommerceDriveMember	[1]
Total	4,386,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneCommerceDriveMember
Accumulated depreciation	1,782,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneCommerceDriveMember	[2]
Office And Office/Flex Buildings [Member] | 102 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1999
Initial Costs, Land	389,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoCommerceDriveMember
Initial Costs, Building and Improvements	1,554,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	556,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	389,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,110,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoCommerceDriveMember	[1]
Total	2,499,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoCommerceDriveMember
Accumulated depreciation	750,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoCommerceDriveMember	[2]
Office And Office/Flex Buildings [Member] | 201 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1986
Acquired	1998
Initial Costs, Land	254,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneCommerceDriveMember
Initial Costs, Building and Improvements	1,694,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	329,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	258,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,019,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneCommerceDriveMember	[1]
Total	2,277,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneCommerceDriveMember
Accumulated depreciation	850,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredOneCommerceDriveMember	[2]
Office And Office/Flex Buildings [Member] | 202 Commerce Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1999
Initial Costs, Land	490,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwoCommerceDriveMember
Initial Costs, Building and Improvements	1,963,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwoCommerceDriveMember
Costs Capitalized Subsequent to Acquisition	384,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwoCommerceDriveMember
Gross Amount at Which Carried at Close of Period, Land	490,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwoCommerceDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,347,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwoCommerceDriveMember	[1]
Total	2,837,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwoCommerceDriveMember
Accumulated depreciation	851,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwoCommerceDriveMember	[2]
Office And Office/Flex Buildings [Member] | 1 Executive Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1998
Initial Costs, Land	226,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveDriveMember
Initial Costs, Building and Improvements	1,453,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveDriveMember
Costs Capitalized Subsequent to Acquisition	739,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveDriveMember
Gross Amount at Which Carried at Close of Period, Land	228,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,190,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveDriveMember	[1]
Total	2,418,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveDriveMember
Accumulated depreciation	873,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneExecutiveDriveMember	[2]
Office And Office/Flex Buildings [Member] | 2 Executive Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	2000
Initial Costs, Land	801,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveDriveMember
Initial Costs, Building and Improvements	3,206,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveDriveMember
Costs Capitalized Subsequent to Acquisition	707,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveDriveMember
Gross Amount at Which Carried at Close of Period, Land	801,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,913,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveDriveMember	[1]
Total	4,714,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveDriveMember
Accumulated depreciation	1,360,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveDriveMember	[2]
Office And Office/Flex Buildings [Member] | 101 Executive Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1998
Initial Costs, Land	241,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveDriveMember
Initial Costs, Building and Improvements	2,262,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveDriveMember
Costs Capitalized Subsequent to Acquisition	807,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveDriveMember
Gross Amount at Which Carried at Close of Period, Land	244,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,066,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveDriveMember	[1]
Total	3,310,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveDriveMember
Accumulated depreciation	1,332,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredOneExecutiveDriveMember	[2]
Office And Office/Flex Buildings [Member] | 102 Executive Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1998
Initial Costs, Land	353,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoExecutiveDriveMember
Initial Costs, Building and Improvements	3,607,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoExecutiveDriveMember
Costs Capitalized Subsequent to Acquisition	420,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoExecutiveDriveMember
Gross Amount at Which Carried at Close of Period, Land	357,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoExecutiveDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,023,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoExecutiveDriveMember	[1]
Total	4,380,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoExecutiveDriveMember
Accumulated depreciation	1,701,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwoExecutiveDriveMember	[2]
Office And Office/Flex Buildings [Member] | 225 Executive Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1998
Initial Costs, Land	323,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyFiveExecutiveDriveMember
Initial Costs, Building and Improvements	2,477,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyFiveExecutiveDriveMember
Costs Capitalized Subsequent to Acquisition	693,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyFiveExecutiveDriveMember
Gross Amount at Which Carried at Close of Period, Land	326,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyFiveExecutiveDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,167,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyFiveExecutiveDriveMember	[1]
Total	3,493,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyFiveExecutiveDriveMember
Accumulated depreciation	1,436,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredTwentyFiveExecutiveDriveMember	[2]
Office And Office/Flex Buildings [Member] | 97 Foster Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1982
Acquired	1998
Initial Costs, Land	208,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NinetySevenFosterRoadMember
Initial Costs, Building and Improvements	1,382,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NinetySevenFosterRoadMember
Costs Capitalized Subsequent to Acquisition	389,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NinetySevenFosterRoadMember
Gross Amount at Which Carried at Close of Period, Land	211,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NinetySevenFosterRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,768,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NinetySevenFosterRoadMember	[1]
Total	1,979,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NinetySevenFosterRoadMember
Accumulated depreciation	823,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NinetySevenFosterRoadMember	[2]
Office And Office/Flex Buildings [Member] | 1507 Lancer Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1995
Acquired	1998
Initial Costs, Land	119,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFiveHundredSevenLancerDriveMember
Initial Costs, Building and Improvements	1,106,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFiveHundredSevenLancerDriveMember
Costs Capitalized Subsequent to Acquisition	209,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFiveHundredSevenLancerDriveMember
Gross Amount at Which Carried at Close of Period, Land	120,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFiveHundredSevenLancerDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,314,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFiveHundredSevenLancerDriveMember	[1]
Total	1,434,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFiveHundredSevenLancerDriveMember
Accumulated depreciation	561,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFiveHundredSevenLancerDriveMember	[2]
Office And Office/Flex Buildings [Member] | 1245 North Church Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1998
Acquired	2001
Initial Costs, Land	691,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortyFiveNorthChurchStreetMember
Initial Costs, Building and Improvements	2,810,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortyFiveNorthChurchStreetMember
Costs Capitalized Subsequent to Acquisition	83,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortyFiveNorthChurchStreetMember
Gross Amount at Which Carried at Close of Period, Land	691,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortyFiveNorthChurchStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,893,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortyFiveNorthChurchStreetMember	[1]
Total	3,584,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortyFiveNorthChurchStreetMember
Accumulated depreciation	997,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortyFiveNorthChurchStreetMember	[2]
Office And Office/Flex Buildings [Member] | 1247 North Church Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1998
Acquired	2001
Initial Costs, Land	805,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortySevenNorthChurchStreetMember
Initial Costs, Building and Improvements	3,269,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortySevenNorthChurchStreetMember
Costs Capitalized Subsequent to Acquisition	117,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortySevenNorthChurchStreetMember
Gross Amount at Which Carried at Close of Period, Land	805,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortySevenNorthChurchStreetMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,386,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortySevenNorthChurchStreetMember	[1]
Total	4,191,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortySevenNorthChurchStreetMember
Accumulated depreciation	1,172,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFortySevenNorthChurchStreetMember	[2]
Office And Office/Flex Buildings [Member] | 1256 North Church Street [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	1998
Initial Costs, Land	354,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFiftySixNorthChurchStreetMember
Initial Costs, Building and Improvements	3,098,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFiftySixNorthChurchStreetMember
Costs Capitalized Subsequent to Acquisition	658,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFiftySixNorthChurchStreetMember
Gross Amount at Which Carried at Close of Period, Land	357,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFiftySixNorthChurchStreetMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,753,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFiftySixNorthChurchStreetMember
Total	4,110,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFiftySixNorthChurchStreetMember
Accumulated depreciation	1,542,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandTwoHundredFiftySixNorthChurchStreetMember	[2]
Office And Office/Flex Buildings [Member] | 840 North Lenola Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1995
Acquired	1998
Initial Costs, Land	329,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyNorthLenolaRoadMember
Initial Costs, Building and Improvements	2,366,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyNorthLenolaRoadMember
Costs Capitalized Subsequent to Acquisition	246,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyNorthLenolaRoadMember
Gross Amount at Which Carried at Close of Period, Land	333,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyNorthLenolaRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,608,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyNorthLenolaRoadMember
Total	2,941,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyNorthLenolaRoadMember
Accumulated depreciation	1,206,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyNorthLenolaRoadMember	[2]
Office And Office/Flex Buildings [Member] | 844 North Lenola Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1995
Acquired	1998
Initial Costs, Land	239,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyFourNorthLenolaRoadMember
Initial Costs, Building and Improvements	1,714,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyFourNorthLenolaRoadMember
Costs Capitalized Subsequent to Acquisition	298,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyFourNorthLenolaRoadMember
Gross Amount at Which Carried at Close of Period, Land	241,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyFourNorthLenolaRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,010,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyFourNorthLenolaRoadMember
Total	2,251,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyFourNorthLenolaRoadMember
Accumulated depreciation	881,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightHundredFortyFourNorthLenolaRoadMember	[2]
Office And Office/Flex Buildings [Member] | 915 North Lenola Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1998
Acquired	2000
Initial Costs, Land	508,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredFifteenNorthLenolaRoadMember
Initial Costs, Building and Improvements	2,034,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredFifteenNorthLenolaRoadMember
Costs Capitalized Subsequent to Acquisition	273,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredFifteenNorthLenolaRoadMember
Gross Amount at Which Carried at Close of Period, Land	508,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredFifteenNorthLenolaRoadMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,307,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredFifteenNorthLenolaRoadMember
Total	2,815,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredFifteenNorthLenolaRoadMember
Accumulated depreciation	980,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_NineHundredFifteenNorthLenolaRoadMember	[2]
Office And Office/Flex Buildings [Member] | 2 Twosome Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2000
Acquired	2001
Initial Costs, Land	701,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoTwosomeDriveMember
Initial Costs, Building and Improvements	2,807,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoTwosomeDriveMember
Costs Capitalized Subsequent to Acquisition	64,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Land	701,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,871,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoTwosomeDriveMember
Total	3,572,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoTwosomeDriveMember
Accumulated depreciation	992,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoTwosomeDriveMember	[2]
Office And Office/Flex Buildings [Member] | 30 Twosome Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1997
Acquired	1998
Initial Costs, Land	234,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyTwosomeDriveMember
Initial Costs, Building and Improvements	1,954,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyTwosomeDriveMember
Costs Capitalized Subsequent to Acquisition	408,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Land	236,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,360,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyTwosomeDriveMember
Total	2,596,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyTwosomeDriveMember
Accumulated depreciation	1,156,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyTwosomeDriveMember	[2]
Office And Office/Flex Buildings [Member] | 31 Twosome Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1998
Acquired	2001
Initial Costs, Land	815,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyOneTwosomeDriveMember
Initial Costs, Building and Improvements	3,276,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyOneTwosomeDriveMember
Costs Capitalized Subsequent to Acquisition	145,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyOneTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Land	815,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyOneTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,421,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyOneTwosomeDriveMember
Total	4,236,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyOneTwosomeDriveMember
Accumulated depreciation	1,211,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThirtyOneTwosomeDriveMember	[2]
Office And Office/Flex Buildings [Member] | 40 Twosome Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1996
Acquired	1998
Initial Costs, Land	297,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyTwosomeDriveMember
Initial Costs, Building and Improvements	2,393,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyTwosomeDriveMember
Costs Capitalized Subsequent to Acquisition	220,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Land	301,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,609,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyTwosomeDriveMember
Total	2,910,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyTwosomeDriveMember
Accumulated depreciation	1,156,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyTwosomeDriveMember	[2]
Office And Office/Flex Buildings [Member] | 41 Twosome Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1998
Acquired	2001
Initial Costs, Land	605,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyOneTwosomeDriveMember
Initial Costs, Building and Improvements	2,459,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyOneTwosomeDriveMember
Costs Capitalized Subsequent to Acquisition	178,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyOneTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Land	605,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyOneTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,637,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyOneTwosomeDriveMember
Total	3,242,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyOneTwosomeDriveMember
Accumulated depreciation	923,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyOneTwosomeDriveMember	[2]
Office And Office/Flex Buildings [Member] | 50 Twosome Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1997
Acquired	1998
Initial Costs, Land	301,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTwosomeDriveMember
Initial Costs, Building and Improvements	2,330,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTwosomeDriveMember
Costs Capitalized Subsequent to Acquisition	98,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Land	304,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTwosomeDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,425,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTwosomeDriveMember
Total	2,729,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTwosomeDriveMember
Accumulated depreciation	1,065,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyTwosomeDriveMember	[2]
Office And Office/Flex Buildings [Member] | 1451 Metropolitan Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1996
Acquired	1998
Initial Costs, Land	203,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredFiftyOneMetropolitanDriveMember
Initial Costs, Building and Improvements	1,189,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredFiftyOneMetropolitanDriveMember
Costs Capitalized Subsequent to Acquisition	74,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredFiftyOneMetropolitanDriveMember
Gross Amount at Which Carried at Close of Period, Land	206,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredFiftyOneMetropolitanDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,260,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredFiftyOneMetropolitanDriveMember
Total	1,466,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredFiftyOneMetropolitanDriveMember
Accumulated depreciation	534,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredFiftyOneMetropolitanDriveMember	[2]
Office And Office/Flex Buildings [Member] | 100 Horizon Center Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1995
Initial Costs, Land	205,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredHorizonCenterBoulevardMember
Initial Costs, Building and Improvements	1,676,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredHorizonCenterBoulevardMember
Costs Capitalized Subsequent to Acquisition	728,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredHorizonCenterBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	323,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredHorizonCenterBoulevardMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,286,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredHorizonCenterBoulevardMember
Total	2,609,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredHorizonCenterBoulevardMember
Accumulated depreciation	934,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredHorizonCenterBoulevardMember	[2]
Office And Office/Flex Buildings [Member] | 200 Horizon Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1991
Acquired	1995
Initial Costs, Land	205,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredHorizonDriveMember
Initial Costs, Building and Improvements	3,027,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredHorizonDriveMember
Costs Capitalized Subsequent to Acquisition	714,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Land	356,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,590,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredHorizonDriveMember
Total	3,946,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredHorizonDriveMember
Accumulated depreciation	1,700,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredHorizonDriveMember	[2]
Office And Office/Flex Buildings [Member] | 300 Horizon Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1995
Initial Costs, Land	379,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredHorizonDriveMember
Initial Costs, Building and Improvements	4,355,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredHorizonDriveMember
Costs Capitalized Subsequent to Acquisition	1,188,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Land	530,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,392,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredHorizonDriveMember
Total	5,922,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredHorizonDriveMember
Accumulated depreciation	2,537,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredHorizonDriveMember	[2]
Office And Office/Flex Buildings [Member] | 500 Horizon Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1995
Initial Costs, Land	379,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredHorizonDriveMember
Initial Costs, Building and Improvements	3,395,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredHorizonDriveMember
Costs Capitalized Subsequent to Acquisition	921,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Land	495,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,200,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredHorizonDriveMember
Total	4,695,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredHorizonDriveMember
Accumulated depreciation	2,029,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredHorizonDriveMember	[2]
Office And Office/Flex Buildings [Member] | 600 Horizon Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2002
Acquired	2002
Initial Costs, Building and Improvements	7,549,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredHorizonDriveMember
Costs Capitalized Subsequent to Acquisition	651,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Land	685,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredHorizonDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,515,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredHorizonDriveMember
Total	8,200,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredHorizonDriveMember
Accumulated depreciation	2,270,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredHorizonDriveMember	[2]
Office And Office/Flex Buildings [Member] | 1325 Campus Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1995
Initial Costs, Land	270,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFiveCampusParkwayMember
Initial Costs, Building and Improvements	2,928,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFiveCampusParkwayMember
Costs Capitalized Subsequent to Acquisition	725,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFiveCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Land	270,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFiveCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,653,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFiveCampusParkwayMember
Total	3,923,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFiveCampusParkwayMember
Accumulated depreciation	1,749,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFiveCampusParkwayMember	[2]
Office And Office/Flex Buildings [Member] | 1340 Campus Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1992
Acquired	1995
Initial Costs, Land	489,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyCampusParkwayMember
Initial Costs, Building and Improvements	4,621,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyCampusParkwayMember
Costs Capitalized Subsequent to Acquisition	1,817,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Land	489,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	6,438,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyCampusParkwayMember
Total	6,927,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyCampusParkwayMember
Accumulated depreciation	3,169,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyCampusParkwayMember	[2]
Office And Office/Flex Buildings [Member] | 1345 Campus Parkway [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1995
Acquired	1997
Initial Costs, Land	1,023,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyFiveCampusParkwayMember
Initial Costs, Building and Improvements	5,703,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyFiveCampusParkwayMember
Costs Capitalized Subsequent to Acquisition	1,703,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyFiveCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Land	1,024,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyFiveCampusParkwayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,405,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyFiveCampusParkwayMember
Total	8,429,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyFiveCampusParkwayMember
Accumulated depreciation	3,526,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredFortyFiveCampusParkwayMember	[2]
Office And Office/Flex Buildings [Member] | 1433 Highway 34 [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	1995
Initial Costs, Land	889,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredThirtyThreeHighwayThirtyFourMember
Initial Costs, Building and Improvements	4,321,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredThirtyThreeHighwayThirtyFourMember
Costs Capitalized Subsequent to Acquisition	1,617,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredThirtyThreeHighwayThirtyFourMember
Gross Amount at Which Carried at Close of Period, Land	889,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredThirtyThreeHighwayThirtyFourMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,938,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredThirtyThreeHighwayThirtyFourMember
Total	6,827,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredThirtyThreeHighwayThirtyFourMember
Accumulated depreciation	2,670,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandFourHundredThirtyThreeHighwayThirtyFourMember	[2]
Office And Office/Flex Buildings [Member] | 1320 Wyckoff Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1986
Acquired	1995
Initial Costs, Land	255,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyWyckoffAvenueMember
Initial Costs, Building and Improvements	1,285,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyWyckoffAvenueMember
Costs Capitalized Subsequent to Acquisition	291,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyWyckoffAvenueMember
Gross Amount at Which Carried at Close of Period, Land	216,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyWyckoffAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,615,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyWyckoffAvenueMember
Total	1,831,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyWyckoffAvenueMember
Accumulated depreciation	833,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyWyckoffAvenueMember	[2]
Office And Office/Flex Buildings [Member] | 1324 Wyckoff Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1995
Initial Costs, Land	230,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFourWyckoffAvenueMember
Initial Costs, Building and Improvements	1,439,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFourWyckoffAvenueMember
Costs Capitalized Subsequent to Acquisition	284,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFourWyckoffAvenueMember
Gross Amount at Which Carried at Close of Period, Land	190,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFourWyckoffAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,763,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFourWyckoffAvenueMember
Total	1,953,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFourWyckoffAvenueMember
Accumulated depreciation	777,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneThousandThreeHundredTwentyFourWyckoffAvenueMember	[2]
Office And Office/Flex Buildings [Member] | 1 Center Court [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1999
Acquired	1999
Initial Costs, Land	270,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneCenterCourtMember
Initial Costs, Building and Improvements	1,824,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneCenterCourtMember
Costs Capitalized Subsequent to Acquisition	493,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneCenterCourtMember
Gross Amount at Which Carried at Close of Period, Land	270,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneCenterCourtMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,317,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneCenterCourtMember
Total	2,587,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneCenterCourtMember
Accumulated depreciation	897,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneCenterCourtMember	[2]
Office And Office/Flex Buildings [Member] | 2 Center Court [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1998
Acquired	1998
Initial Costs, Land	191,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCenterCourtMember
Costs Capitalized Subsequent to Acquisition	2,646,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCenterCourtMember
Gross Amount at Which Carried at Close of Period, Land	191,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCenterCourtMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,646,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCenterCourtMember
Total	2,837,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCenterCourtMember
Accumulated depreciation	1,037,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoCenterCourtMember	[2]
Office And Office/Flex Buildings [Member] | 11 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1995
Initial Costs, Land	586,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceWayMember
Initial Costs, Building and Improvements	2,986,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceWayMember
Costs Capitalized Subsequent to Acquisition	1,000,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	586,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,986,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceWayMember
Total	4,572,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceWayMember
Accumulated depreciation	1,879,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 20 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1992
Acquired	1995
Initial Costs, Land	516,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceWayMember
Initial Costs, Building and Improvements	3,108,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceWayMember
Costs Capitalized Subsequent to Acquisition	111,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	516,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,219,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceWayMember
Total	3,735,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceWayMember
Accumulated depreciation	1,514,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 29 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1995
Initial Costs, Land	586,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyNineCommerceWayMember
Initial Costs, Building and Improvements	3,092,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyNineCommerceWayMember
Costs Capitalized Subsequent to Acquisition	858,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyNineCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	586,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyNineCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,950,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyNineCommerceWayMember
Total	4,536,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyNineCommerceWayMember
Accumulated depreciation	1,642,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwentyNineCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 40 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1995
Initial Costs, Land	516,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyCommerceWayMember
Initial Costs, Building and Improvements	3,260,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyCommerceWayMember
Costs Capitalized Subsequent to Acquisition	1,510,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	516,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,770,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyCommerceWayMember
Total	5,286,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyCommerceWayMember
Accumulated depreciation	2,058,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 45 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1992
Acquired	1995
Initial Costs, Land	536,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyFiveCommerceWayMember
Initial Costs, Building and Improvements	3,379,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyFiveCommerceWayMember
Costs Capitalized Subsequent to Acquisition	576,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyFiveCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	536,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyFiveCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,955,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyFiveCommerceWayMember
Total	4,491,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyFiveCommerceWayMember
Accumulated depreciation	2,023,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FortyFiveCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 60 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1995
Initial Costs, Land	526,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyCommerceWayMember
Initial Costs, Building and Improvements	3,257,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyCommerceWayMember
Costs Capitalized Subsequent to Acquisition	635,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	526,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,892,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyCommerceWayMember
Total	4,418,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyCommerceWayMember
Accumulated depreciation	2,027,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixtyCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 80 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1996
Acquired	1996
Initial Costs, Land	227,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyCommerceWayMember
Costs Capitalized Subsequent to Acquisition	1,357,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	227,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,357,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyCommerceWayMember
Total	1,584,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyCommerceWayMember
Accumulated depreciation	597,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 100 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1996
Acquired	1996
Initial Costs, Land	226,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCommerceWayMember
Costs Capitalized Subsequent to Acquisition	1,356,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	226,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,356,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCommerceWayMember
Total	1,582,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCommerceWayMember
Accumulated depreciation	596,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 120 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1994
Acquired	1995
Initial Costs, Land	228,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyCommerceWayMember
Costs Capitalized Subsequent to Acquisition	1,290,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	229,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,289,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyCommerceWayMember
Total	1,518,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyCommerceWayMember
Accumulated depreciation	617,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 140 Commerce Way [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1994
Acquired	1995
Initial Costs, Land	229,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyCommerceWayMember
Costs Capitalized Subsequent to Acquisition	1,288,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Land	228,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyCommerceWayMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,289,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyCommerceWayMember
Total	1,517,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyCommerceWayMember
Accumulated depreciation	617,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFortyCommerceWayMember	[2]
Office And Office/Flex Buildings [Member] | 11 Clearbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1974
Acquired	1997
Initial Costs, Land	149,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenClearbrookRoadMember
Initial Costs, Building and Improvements	2,159,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenClearbrookRoadMember
Costs Capitalized Subsequent to Acquisition	529,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenClearbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	149,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenClearbrookRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,688,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenClearbrookRoadMember
Total	2,837,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenClearbrookRoadMember
Accumulated depreciation	1,129,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenClearbrookRoadMember	[2]
Office And Office/Flex Buildings [Member] | 75 Clearbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1997
Initial Costs, Land	2,314,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveClearbrookRoadMember
Initial Costs, Building and Improvements	4,716,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveClearbrookRoadMember
Costs Capitalized Subsequent to Acquisition	107,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveClearbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	2,314,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveClearbrookRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,823,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveClearbrookRoadMember
Total	7,137,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveClearbrookRoadMember
Accumulated depreciation	2,189,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventyFiveClearbrookRoadMember	[2]
Office And Office/Flex Buildings [Member] | 125 Clearbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2002
Acquired	2002
Initial Costs, Land	1,055,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveClearbrookRoadMember
Initial Costs, Building and Improvements	3,676,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveClearbrookRoadMember
Costs Capitalized Subsequent to Acquisition	(515,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveClearbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	1,055,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveClearbrookRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,161,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveClearbrookRoadMember
Total	4,216,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveClearbrookRoadMember
Accumulated depreciation	947,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredTwentyFiveClearbrookRoadMember	[2]
Office And Office/Flex Buildings [Member] | 150 Clearbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1975
Acquired	1997
Initial Costs, Land	497,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyClearbrookRoadMember
Initial Costs, Building and Improvements	7,030,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyClearbrookRoadMember
Costs Capitalized Subsequent to Acquisition	2,211,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyClearbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	497,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyClearbrookRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	9,241,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyClearbrookRoadMember
Total	9,738,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyClearbrookRoadMember
Accumulated depreciation	3,820,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredFiftyClearbrookRoadMember	[2]
Office And Office/Flex Buildings [Member] | 175 Clearbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1973
Acquired	1997
Initial Costs, Land	655,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredSeventyFiveClearbrookRoadMember
Initial Costs, Building and Improvements	7,473,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredSeventyFiveClearbrookRoadMember
Costs Capitalized Subsequent to Acquisition	676,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredSeventyFiveClearbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	655,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredSeventyFiveClearbrookRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,149,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredSeventyFiveClearbrookRoadMember
Total	8,804,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredSeventyFiveClearbrookRoadMember
Accumulated depreciation	3,946,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredSeventyFiveClearbrookRoadMember	[2]
Office And Office/Flex Buildings [Member] | 200 Clearbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1974
Acquired	1997
Initial Costs, Land	579,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredClearbrookRoadMember
Initial Costs, Building and Improvements	6,620,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredClearbrookRoadMember
Costs Capitalized Subsequent to Acquisition	1,778,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredClearbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	579,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredClearbrookRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,398,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredClearbrookRoadMember
Total	8,977,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredClearbrookRoadMember
Accumulated depreciation	3,573,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredClearbrookRoadMember	[2]
Office And Office/Flex Buildings [Member] | 250 Clearbrook Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1973
Acquired	1997
Initial Costs, Land	867,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyClearbrookRoadMember
Initial Costs, Building and Improvements	8,647,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyClearbrookRoadMember
Costs Capitalized Subsequent to Acquisition	1,685,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyClearbrookRoadMember
Gross Amount at Which Carried at Close of Period, Land	867,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyClearbrookRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	10,332,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyClearbrookRoadMember
Total	11,199,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyClearbrookRoadMember
Accumulated depreciation	4,308,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredFiftyClearbrookRoadMember	[2]
Office And Office/Flex Buildings [Member] | 50 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1969
Acquired	1997
Initial Costs, Land	237,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyExecutiveBoulevardMember
Initial Costs, Building and Improvements	2,617,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	398,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	237,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,015,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyExecutiveBoulevardMember
Total	3,252,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyExecutiveBoulevardMember
Accumulated depreciation	1,308,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiftyExecutiveBoulevardMember	[2]
Office And Office/Flex Buildings [Member] | 77 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1977
Acquired	1997
Initial Costs, Land	34,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventySevenExecutiveBoulevardMember
Initial Costs, Building and Improvements	1,104,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventySevenExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	212,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventySevenExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	34,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventySevenExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,316,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventySevenExecutiveBoulevardMember
Total	1,350,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventySevenExecutiveBoulevardMember
Accumulated depreciation	630,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SeventySevenExecutiveBoulevardMember	[2]
Office And Office/Flex Buildings [Member] | 85 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1968
Acquired	1997
Initial Costs, Land	155,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveExecutiveBoulevardMember
Initial Costs, Building and Improvements	2,507,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	691,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	155,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,198,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveExecutiveBoulevardMember
Total	3,353,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveExecutiveBoulevardMember
Accumulated depreciation	1,503,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightyFiveExecutiveBoulevardMember	[2]
Office And Office/Flex Buildings [Member] | 300 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1970
Acquired	1997
Initial Costs, Land	460,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredExecutiveBoulevardMember
Initial Costs, Building and Improvements	3,609,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	275,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	460,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,884,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredExecutiveBoulevardMember
Total	4,344,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredExecutiveBoulevardMember
Accumulated depreciation	1,680,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredExecutiveBoulevardMember	[2]
Office And Office/Flex Buildings [Member] | 350 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1970
Acquired	1997
Initial Costs, Land	100,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFiftyExecutiveBoulevardMember
Initial Costs, Building and Improvements	1,793,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFiftyExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	175,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFiftyExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	100,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFiftyExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,968,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFiftyExecutiveBoulevardMember
Total	2,068,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFiftyExecutiveBoulevardMember
Accumulated depreciation	871,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredFiftyExecutiveBoulevardMember
Office And Office/Flex Buildings [Member] | 399 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1962
Acquired	1997
Initial Costs, Land	531,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyNineExecutiveBoulevardMember
Initial Costs, Building and Improvements	7,191,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyNineExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	163,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyNineExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	531,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyNineExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,354,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyNineExecutiveBoulevardMember
Total	7,885,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyNineExecutiveBoulevardMember
Accumulated depreciation	3,310,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeHundredNinetyNineExecutiveBoulevardMember
Office And Office/Flex Buildings [Member] | 400 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1970
Acquired	1997
Initial Costs, Land	2,202,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredExecutiveBoulevardMember
Initial Costs, Building and Improvements	1,846,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	841,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	2,202,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,687,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredExecutiveBoulevardMember
Total	4,889,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredExecutiveBoulevardMember
Accumulated depreciation	1,263,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredExecutiveBoulevardMember
Office And Office/Flex Buildings [Member] | 500 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1970
Acquired	1997
Initial Costs, Land	258,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredExecutiveBoulevardMember
Initial Costs, Building and Improvements	4,183,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	465,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	258,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,648,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredExecutiveBoulevardMember
Total	4,906,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredExecutiveBoulevardMember
Accumulated depreciation	2,148,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredExecutiveBoulevardMember
Office And Office/Flex Buildings [Member] | 525 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1972
Acquired	1997
Initial Costs, Land	345,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredTwentyFiveExecutiveBoulevardMember
Initial Costs, Building and Improvements	5,499,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredTwentyFiveExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	834,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredTwentyFiveExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	345,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredTwentyFiveExecutiveBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	6,333,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredTwentyFiveExecutiveBoulevardMember
Total	6,678,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredTwentyFiveExecutiveBoulevardMember
Accumulated depreciation	2,855,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredTwentyFiveExecutiveBoulevardMember
Office And Office/Flex Buildings [Member] | 1 Westchester Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1967
Acquired	1997
Initial Costs, Land	199,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWestchesterPlazaMember
Initial Costs, Building and Improvements	2,023,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWestchesterPlazaMember
Costs Capitalized Subsequent to Acquisition	469,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWestchesterPlazaMember
Gross Amount at Which Carried at Close of Period, Land	199,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWestchesterPlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,492,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWestchesterPlazaMember
Total	2,691,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWestchesterPlazaMember
Accumulated depreciation	1,197,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWestchesterPlazaMember
Office And Office/Flex Buildings [Member] | 2 Westchester Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1968
Acquired	1997
Initial Costs, Land	234,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWestchesterPlazaMember
Initial Costs, Building and Improvements	2,726,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWestchesterPlazaMember
Costs Capitalized Subsequent to Acquisition	701,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWestchesterPlazaMember
Gross Amount at Which Carried at Close of Period, Land	234,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWestchesterPlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,427,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWestchesterPlazaMember
Total	3,661,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWestchesterPlazaMember
Accumulated depreciation	1,417,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWestchesterPlazaMember
Office And Office/Flex Buildings [Member] | 3 Westchester Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1969
Acquired	1997
Initial Costs, Land	655,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWestchesterPlazaMember
Initial Costs, Building and Improvements	7,936,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWestchesterPlazaMember
Costs Capitalized Subsequent to Acquisition	1,114,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWestchesterPlazaMember
Gross Amount at Which Carried at Close of Period, Land	655,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWestchesterPlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	9,050,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWestchesterPlazaMember
Total	9,705,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWestchesterPlazaMember
Accumulated depreciation	4,202,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWestchesterPlazaMember
Office And Office/Flex Buildings [Member] | 4 Westchester Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1969
Acquired	1997
Initial Costs, Land	320,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWestchesterPlazaMember
Initial Costs, Building and Improvements	3,729,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWestchesterPlazaMember
Costs Capitalized Subsequent to Acquisition	1,038,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWestchesterPlazaMember
Gross Amount at Which Carried at Close of Period, Land	320,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWestchesterPlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,767,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWestchesterPlazaMember
Total	5,087,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWestchesterPlazaMember
Accumulated depreciation	2,229,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWestchesterPlazaMember
Office And Office/Flex Buildings [Member] | 5 Westchester Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1969
Acquired	1997
Initial Costs, Land	118,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWestchesterPlazaMember
Initial Costs, Building and Improvements	1,949,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWestchesterPlazaMember
Costs Capitalized Subsequent to Acquisition	462,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWestchesterPlazaMember
Gross Amount at Which Carried at Close of Period, Land	118,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWestchesterPlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,411,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWestchesterPlazaMember
Total	2,529,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWestchesterPlazaMember
Accumulated depreciation	1,219,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWestchesterPlazaMember
Office And Office/Flex Buildings [Member] | 6 Westchester Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1968
Acquired	1997
Initial Costs, Land	164,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWestchesterPlazaMember
Initial Costs, Building and Improvements	1,998,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWestchesterPlazaMember
Costs Capitalized Subsequent to Acquisition	155,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWestchesterPlazaMember
Gross Amount at Which Carried at Close of Period, Land	164,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWestchesterPlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,153,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWestchesterPlazaMember
Total	2,317,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWestchesterPlazaMember
Accumulated depreciation	952,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWestchesterPlazaMember
Office And Office/Flex Buildings [Member] | 7 Westchester Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1972
Acquired	1997
Initial Costs, Land	286,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenWestchesterPlazaMember
Initial Costs, Building and Improvements	4,321,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenWestchesterPlazaMember
Costs Capitalized Subsequent to Acquisition	682,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenWestchesterPlazaMember
Gross Amount at Which Carried at Close of Period, Land	286,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenWestchesterPlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,003,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenWestchesterPlazaMember
Total	5,289,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenWestchesterPlazaMember
Accumulated depreciation	2,044,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenWestchesterPlazaMember
Office And Office/Flex Buildings [Member] | 8 Westchester Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1971
Acquired	1997
Initial Costs, Land	447,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightWestchesterPlazaMember
Initial Costs, Building and Improvements	5,262,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightWestchesterPlazaMember
Costs Capitalized Subsequent to Acquisition	1,932,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightWestchesterPlazaMember
Gross Amount at Which Carried at Close of Period, Land	447,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightWestchesterPlazaMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,194,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightWestchesterPlazaMember
Total	7,641,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightWestchesterPlazaMember
Accumulated depreciation	2,875,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightWestchesterPlazaMember
Office And Office/Flex Buildings [Member] | 200 Saw Mill River Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1965
Acquired	1997
Initial Costs, Land	353,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredSawMillRiverRoadMember
Initial Costs, Building and Improvements	3,353,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredSawMillRiverRoadMember
Costs Capitalized Subsequent to Acquisition	608,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredSawMillRiverRoadMember
Gross Amount at Which Carried at Close of Period, Land	353,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredSawMillRiverRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,961,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredSawMillRiverRoadMember
Total	4,314,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredSawMillRiverRoadMember
Accumulated depreciation	1,786,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredSawMillRiverRoadMember
Office And Office/Flex Buildings [Member] | 4 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1997
Initial Costs, Land	363,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSkylineDriveMember
Initial Costs, Building and Improvements	7,513,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	2,790,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	363,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	10,303,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSkylineDriveMember
Total	10,666,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSkylineDriveMember
Accumulated depreciation	4,874,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourSkylineDriveMember
Office And Office/Flex Buildings [Member] | 5 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1980
Acquired	2001
Initial Costs, Land	2,219,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSkylineDriveMember
Initial Costs, Building and Improvements	8,916,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	2,001,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	2,219,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	10,917,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSkylineDriveMember
Total	13,136,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSkylineDriveMember
Accumulated depreciation	4,588,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveSkylineDriveMember
Office And Office/Flex Buildings [Member] | 6 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1980
Acquired	2001
Initial Costs, Land	740,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixSkylineDriveMember
Initial Costs, Building and Improvements	2,971,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	1,044,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	740,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,015,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixSkylineDriveMember
Total	4,755,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixSkylineDriveMember
Accumulated depreciation	2,142,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixSkylineDriveMember
Office And Office/Flex Buildings [Member] | 8 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	1997
Initial Costs, Land	212,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightSkylineDriveMember
Initial Costs, Building and Improvements	4,410,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	951,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	212,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,361,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightSkylineDriveMember
Total	5,573,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightSkylineDriveMember
Accumulated depreciation	2,577,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_EightSkylineDriveMember
Office And Office/Flex Buildings [Member] | 10 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1985
Acquired	1997
Initial Costs, Land	134,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenSkylineDriveMember
Initial Costs, Building and Improvements	2,799,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	732,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	134,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,531,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenSkylineDriveMember
Total	3,665,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenSkylineDriveMember
Accumulated depreciation	1,757,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TenSkylineDriveMember
Office And Office/Flex Buildings [Member] | 11 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989	[3]
Acquired	1997	[3]
Initial Costs, Building and Improvements	4,788,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenSkylineDriveMember	[3]
Costs Capitalized Subsequent to Acquisition	760,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenSkylineDriveMember	[3]
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,548,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenSkylineDriveMember	[3]
Total	5,548,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenSkylineDriveMember	[3]
Accumulated depreciation	2,252,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ElevenSkylineDriveMember	[3]
Office And Office/Flex Buildings [Member] | 12 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1999
Acquired	1999
Initial Costs, Land	1,562,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveSkylineDriveMember
Initial Costs, Building and Improvements	3,254,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	21,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Land	1,320,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveSkylineDriveMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,517,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveSkylineDriveMember
Total	4,837,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveSkylineDriveMember
Accumulated depreciation	1,368,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwelveSkylineDriveMember
Office And Office/Flex Buildings [Member] | 15 Skyline Drive [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1989
Acquired	1997
Initial Costs, Building and Improvements	7,449,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenSkylineDriveMember
Costs Capitalized Subsequent to Acquisition	628,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenSkylineDriveMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	8,077,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenSkylineDriveMember
Total	8,077,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenSkylineDriveMember
Accumulated depreciation	3,544,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FifteenSkylineDriveMember
Office And Office/Flex Buildings [Member] | 100 Corporate Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1997
Initial Costs, Land	602,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCorporateBoulevardMember
Initial Costs, Building and Improvements	9,910,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCorporateBoulevardMember
Costs Capitalized Subsequent to Acquisition	1,519,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCorporateBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	602,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCorporateBoulevardMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	11,429,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCorporateBoulevardMember
Total	12,031,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCorporateBoulevardMember
Accumulated depreciation	5,126,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneHundredCorporateBoulevardMember
Office And Office/Flex Buildings [Member] | 200 Corporate Boulevard South [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1997
Initial Costs, Land	502,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredCorporateBoulevardSouthMember
Initial Costs, Building and Improvements	7,575,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredCorporateBoulevardSouthMember
Costs Capitalized Subsequent to Acquisition	1,538,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredCorporateBoulevardSouthMember
Gross Amount at Which Carried at Close of Period, Land	502,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredCorporateBoulevardSouthMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	9,113,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredCorporateBoulevardSouthMember
Total	9,615,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredCorporateBoulevardSouthMember
Accumulated depreciation	4,107,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredCorporateBoulevardSouthMember
Office And Office/Flex Buildings [Member] | 4 Executive Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1986
Acquired	1997
Initial Costs, Land	584,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourExecutivePlazaMember
Initial Costs, Building and Improvements	6,134,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourExecutivePlazaMember
Costs Capitalized Subsequent to Acquisition	942,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourExecutivePlazaMember
Gross Amount at Which Carried at Close of Period, Land	584,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourExecutivePlazaMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	7,076,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourExecutivePlazaMember
Total	7,660,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourExecutivePlazaMember
Accumulated depreciation	3,285,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourExecutivePlazaMember
Office And Office/Flex Buildings [Member] | 6 Executive Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1987
Acquired	1997
Initial Costs, Land	546,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixExecutivePlazaMember
Initial Costs, Building and Improvements	7,246,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixExecutivePlazaMember
Costs Capitalized Subsequent to Acquisition	2,146,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixExecutivePlazaMember
Gross Amount at Which Carried at Close of Period, Land	546,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixExecutivePlazaMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	9,392,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixExecutivePlazaMember
Total	9,938,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixExecutivePlazaMember
Accumulated depreciation	3,882,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixExecutivePlazaMember
Office And Office/Flex Buildings [Member] | 1 Odell Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1980
Acquired	1997
Initial Costs, Land	1,206,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneOdellPlazaMember
Initial Costs, Building and Improvements	6,815,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneOdellPlazaMember
Costs Capitalized Subsequent to Acquisition	2,353,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneOdellPlazaMember
Gross Amount at Which Carried at Close of Period, Land	1,206,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneOdellPlazaMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	9,168,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneOdellPlazaMember
Total	10,374,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneOdellPlazaMember
Accumulated depreciation	3,895,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneOdellPlazaMember
Office And Office/Flex Buildings [Member] | 5 Odell Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1983
Acquired	1997
Initial Costs, Land	331,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveOdellPlazaMember
Initial Costs, Building and Improvements	2,988,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveOdellPlazaMember
Costs Capitalized Subsequent to Acquisition	894,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveOdellPlazaMember
Gross Amount at Which Carried at Close of Period, Land	331,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveOdellPlazaMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,882,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveOdellPlazaMember
Total	4,213,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveOdellPlazaMember
Accumulated depreciation	1,994,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveOdellPlazaMember
Office And Office/Flex Buildings [Member] | 7 Odell Plaza [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	1997
Initial Costs, Land	419,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenOdellPlazaMember
Initial Costs, Building and Improvements	4,418,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenOdellPlazaMember
Costs Capitalized Subsequent to Acquisition	1,045,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenOdellPlazaMember
Gross Amount at Which Carried at Close of Period, Land	419,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenOdellPlazaMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,463,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenOdellPlazaMember
Total	5,882,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenOdellPlazaMember
Accumulated depreciation	2,281,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenOdellPlazaMember
Office And Office/Flex Buildings [Member] | 419 West Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1986
Acquired	1997
Initial Costs, Land	4,538,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredNineteenWestAvenueMember
Initial Costs, Building and Improvements	9,246,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredNineteenWestAvenueMember
Costs Capitalized Subsequent to Acquisition	1,295,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredNineteenWestAvenueMember
Gross Amount at Which Carried at Close of Period, Land	4,538,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredNineteenWestAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	10,541,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredNineteenWestAvenueMember
Total	15,079,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredNineteenWestAvenueMember
Accumulated depreciation	4,956,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourHundredNineteenWestAvenueMember
Office And Office/Flex Buildings [Member] | 500 West Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	1997
Initial Costs, Land	415,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredWestAvenueMember
Initial Costs, Building and Improvements	1,679,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredWestAvenueMember
Costs Capitalized Subsequent to Acquisition	615,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredWestAvenueMember
Gross Amount at Which Carried at Close of Period, Land	415,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredWestAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,294,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredWestAvenueMember
Total	2,709,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredWestAvenueMember
Accumulated depreciation	818,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredWestAvenueMember
Office And Office/Flex Buildings [Member] | 550 West Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1990
Acquired	1997
Initial Costs, Land	1,975,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredFiftyWestAvenueMember
Initial Costs, Building and Improvements	3,856,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredFiftyWestAvenueMember
Costs Capitalized Subsequent to Acquisition	185,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredFiftyWestAvenueMember
Gross Amount at Which Carried at Close of Period, Land	1,975,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredFiftyWestAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	4,041,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredFiftyWestAvenueMember
Total	6,016,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredFiftyWestAvenueMember
Accumulated depreciation	1,846,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredFiftyWestAvenueMember
Office And Office/Flex Buildings [Member] | 600 West Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1999
Acquired	1999
Initial Costs, Land	2,305,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredWestAvenueMember
Initial Costs, Building and Improvements	2,863,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredWestAvenueMember
Costs Capitalized Subsequent to Acquisition	866,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredWestAvenueMember
Gross Amount at Which Carried at Close of Period, Land	2,305,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredWestAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,729,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredWestAvenueMember
Total	6,034,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredWestAvenueMember
Accumulated depreciation	1,445,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredWestAvenueMember
Office And Office/Flex Buildings [Member] | 650 West Avenue [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1998
Acquired	1998
Initial Costs, Land	1,328,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyWestAvenueMember
Costs Capitalized Subsequent to Acquisition	3,503,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyWestAvenueMember
Gross Amount at Which Carried at Close of Period, Land	1,328,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyWestAvenueMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	3,503,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyWestAvenueMember
Total	4,831,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyWestAvenueMember
Accumulated depreciation	1,438,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_OfficeAndOfficeFlexBuildingsMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixHundredFiftyWestAvenueMember
Industrial/Warehouse Facilities [Member] | 1 Warehouse Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1957
Acquired	1997
Initial Costs, Land	3,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWarehouseLaneMember
Initial Costs, Building and Improvements	268,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWarehouseLaneMember
Costs Capitalized Subsequent to Acquisition	233,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWarehouseLaneMember
Gross Amount at Which Carried at Close of Period, Land	3,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWarehouseLaneMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	501,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWarehouseLaneMember
Total	504,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWarehouseLaneMember
Accumulated depreciation	221,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneWarehouseLaneMember
Industrial/Warehouse Facilities [Member] | 2 Warehouse Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1957
Acquired	1997
Initial Costs, Land	4,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWarehouseLaneMember
Initial Costs, Building and Improvements	672,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWarehouseLaneMember
Costs Capitalized Subsequent to Acquisition	122,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWarehouseLaneMember
Gross Amount at Which Carried at Close of Period, Land	4,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWarehouseLaneMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	794,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWarehouseLaneMember
Total	798,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWarehouseLaneMember
Accumulated depreciation	363,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoWarehouseLaneMember
Industrial/Warehouse Facilities [Member] | 3 Warehouse Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1957	[3]
Acquired	1997	[3]
Initial Costs, Land	21,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWarehouseLaneMember	[3]
Initial Costs, Building and Improvements	1,948,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWarehouseLaneMember	[3]
Costs Capitalized Subsequent to Acquisition	363,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWarehouseLaneMember	[3]
Gross Amount at Which Carried at Close of Period, Land	21,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWarehouseLaneMember	[1],[3]
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,311,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWarehouseLaneMember	[3]
Total	2,332,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWarehouseLaneMember	[3]
Accumulated depreciation	1,114,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ThreeWarehouseLaneMember	[3]
Industrial/Warehouse Facilities [Member] | 4 Warehouse Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1957
Acquired	1997
Initial Costs, Land	84,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWarehouseLaneMember
Initial Costs, Building and Improvements	13,393,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWarehouseLaneMember
Costs Capitalized Subsequent to Acquisition	5,155,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWarehouseLaneMember
Gross Amount at Which Carried at Close of Period, Land	85,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWarehouseLaneMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	18,547,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWarehouseLaneMember
Total	18,632,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWarehouseLaneMember
Accumulated depreciation	8,563,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FourWarehouseLaneMember
Industrial/Warehouse Facilities [Member] | 5 Warehouse Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1957	[3]
Acquired	1997	[3]
Initial Costs, Land	19,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWarehouseLaneMember	[3]
Initial Costs, Building and Improvements	4,804,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWarehouseLaneMember	[3]
Costs Capitalized Subsequent to Acquisition	939,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWarehouseLaneMember	[3]
Gross Amount at Which Carried at Close of Period, Land	19,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWarehouseLaneMember	[1],[3]
Gross Amount at Which Carried at Close of Period, Building and Improvements	5,743,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWarehouseLaneMember	[3]
Total	5,762,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWarehouseLaneMember	[3]
Accumulated depreciation	2,551,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveWarehouseLaneMember	[3]
Industrial/Warehouse Facilities [Member] | 6 Warehouse Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1982
Acquired	1997
Initial Costs, Land	10,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWarehouseLaneMember
Initial Costs, Building and Improvements	4,419,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWarehouseLaneMember
Costs Capitalized Subsequent to Acquisition	2,445,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWarehouseLaneMember
Gross Amount at Which Carried at Close of Period, Land	10,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWarehouseLaneMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	6,864,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWarehouseLaneMember
Total	6,874,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWarehouseLaneMember
Accumulated depreciation	2,592,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_IndustrialAndWarehouseFacilitiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SixWarehouseLaneMember
Retail [Member] | 230 White Plains Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Acquired	1997
Initial Costs, Land	124,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyWhitePlainsRoadMember
Initial Costs, Building and Improvements	1,845,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyWhitePlainsRoadMember
Costs Capitalized Subsequent to Acquisition	107,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyWhitePlainsRoadMember
Gross Amount at Which Carried at Close of Period, Land	124,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyWhitePlainsRoadMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	1,952,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyWhitePlainsRoadMember
Total	2,076,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyWhitePlainsRoadMember
Accumulated depreciation	868,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoHundredThirtyWhitePlainsRoadMember
Retail [Member] | 2 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1986
Acquired	1997
Initial Costs, Land	89,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveBoulevardMember
Initial Costs, Building and Improvements	2,439,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveBoulevardMember
Costs Capitalized Subsequent to Acquisition	100,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	89,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	2,539,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveBoulevardMember
Total	2,628,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveBoulevardMember
Accumulated depreciation	1,113,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_RetailSiteMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_TwoExecutiveBoulevardMember
Land Lease [Member] | 700 Executive Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Acquired	1997
Initial Costs, Land	970,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_LandLeaseMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredExecutiveBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	970,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_LandLeaseMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredExecutiveBoulevardMember	[1]
Total	970,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_LandLeaseMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_SevenHundredExecutiveBoulevardMember
Land Lease [Member] | 1 Enterprise Boulevard [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Acquired	1997
Initial Costs, Land	1,379,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_LandLeaseMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneEnterpriseBoulevardMember
Costs Capitalized Subsequent to Acquisition	1,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_LandLeaseMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneEnterpriseBoulevardMember
Gross Amount at Which Carried at Close of Period, Land	1,380,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_LandLeaseMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneEnterpriseBoulevardMember	[1]
Total	1,380,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_LandLeaseMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_OneEnterpriseBoulevardMember
Land [Member] | Capital Office Park Parcel A [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Acquired	2009
Initial Costs, Land	840,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_LandMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_CapitalOfficeParkParcelaMember
Costs Capitalized Subsequent to Acquisition	7,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_LandMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_CapitalOfficeParkParcelaMember
Gross Amount at Which Carried at Close of Period, Land	847,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_LandMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_CapitalOfficeParkParcelaMember
Total	847,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_LandMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_CapitalOfficeParkParcelaMember
Multi-Family Properties [Member] | Richmond Court [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1997
Acquired	2013
Initial Costs, Land	2,992,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RichmondCourtMember
Initial Costs, Building and Improvements	13,534,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RichmondCourtMember
Costs Capitalized Subsequent to Acquisition	50,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RichmondCourtMember
Gross Amount at Which Carried at Close of Period, Land	2,992,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RichmondCourtMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	13,584,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RichmondCourtMember
Total	16,576,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RichmondCourtMember
Accumulated depreciation	350,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RichmondCourtMember	[2]
Multi-Family Properties [Member] | Riverwatch Commons [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1995
Acquired	2013
Initial Costs, Land	4,169,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RiverwatchCommonsMember
Initial Costs, Building and Improvements	18,974,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RiverwatchCommonsMember
Costs Capitalized Subsequent to Acquisition	59,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RiverwatchCommonsMember
Gross Amount at Which Carried at Close of Period, Land	4,169,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RiverwatchCommonsMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	19,033,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RiverwatchCommonsMember
Total	23,202,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RiverwatchCommonsMember
Accumulated depreciation	491,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_RiverwatchCommonsMember	[2]
Multi-Family Properties [Member] | Park Square [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2011
Acquired	2013
Related Encumbrances	27,500,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ParkSquareMember
Initial Costs, Land	4,000,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ParkSquareMember
Initial Costs, Building and Improvements	40,670,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ParkSquareMember
Costs Capitalized Subsequent to Acquisition	21,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ParkSquareMember
Gross Amount at Which Carried at Close of Period, Land	4,000,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ParkSquareMember	[1]
Gross Amount at Which Carried at Close of Period, Building and Improvements	40,691,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ParkSquareMember
Total	44,691,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ParkSquareMember
Accumulated depreciation	1,132,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ParkSquareMember	[2]
Multi-Family Properties [Member] | Alterra At Overlook Ridge IA [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2004
Acquired	2013
Initial Costs, Land	9,042,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIMember
Initial Costs, Building and Improvements	50,671,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIMember
Costs Capitalized Subsequent to Acquisition	357,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIMember
Gross Amount at Which Carried at Close of Period, Land	9,042,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	51,028,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIMember
Total	60,070,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIMember
Accumulated depreciation	2,535,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIMember
Multi-Family Properties [Member] | Alterra at Overlook Ridge II [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2008
Acquired	2013
Initial Costs, Land	12,055,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIIMember
Initial Costs, Building and Improvements	71,409,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIIMember
Costs Capitalized Subsequent to Acquisition	345,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIIMember
Gross Amount at Which Carried at Close of Period, Land	12,055,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIIMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	71,754,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIIMember
Total	83,809,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIIMember
Accumulated depreciation	3,332,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_AlterraAtOverlookRidgeIIMember
Multi-Family Properties [Member] | Apartments At Andover Place [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1988
Acquired	2014
Initial Costs, Land	8,535,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ApartmentsAtAndoverPlaceMember
Initial Costs, Building and Improvements	27,609,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ApartmentsAtAndoverPlaceMember
Costs Capitalized Subsequent to Acquisition	154,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ApartmentsAtAndoverPlaceMember
Gross Amount at Which Carried at Close of Period, Land	8,534,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ApartmentsAtAndoverPlaceMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	27,764,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ApartmentsAtAndoverPlaceMember
Total	36,298,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ApartmentsAtAndoverPlaceMember
Accumulated depreciation	696,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_MultipleFamilyPropertiesMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_ApartmentsAtAndoverPlaceMember
Other Property [Member] | 500 Avenue At Port Imperial [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2013
Acquired	2013
Related Encumbrances	36,600,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OtherPropertyMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredAvenueAtPortImperialMember
Initial Costs, Land	13,099,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OtherPropertyMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredAvenueAtPortImperialMember
Initial Costs, Building and Improvements	56,669,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OtherPropertyMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredAvenueAtPortImperialMember
Costs Capitalized Subsequent to Acquisition	(21,005,000)us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OtherPropertyMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredAvenueAtPortImperialMember
Gross Amount at Which Carried at Close of Period, Land	13,099,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OtherPropertyMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredAvenueAtPortImperialMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	35,664,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OtherPropertyMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredAvenueAtPortImperialMember
Total	48,763,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OtherPropertyMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredAvenueAtPortImperialMember
Accumulated depreciation	1,282,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_OtherPropertyMember / us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis = cli_FiveHundredAvenueAtPortImperialMember	[2]
Projects Under Development And Developable Land [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Related Encumbrances	68,359,000us-gaap_RealEstateAndAccumulatedDepreciationAmountOfEncumbrances / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_ProjectsUnderDevelopmentAndDevelopableLandMember
Initial Costs, Land	174,581,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_ProjectsUnderDevelopmentAndDevelopableLandMember
Initial Costs, Building and Improvements	182,061,000us-gaap_RealEstateAndAccumulatedDepreciationInitialCostOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_ProjectsUnderDevelopmentAndDevelopableLandMember
Gross Amount at Which Carried at Close of Period, Land	174,581,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfLand / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_ProjectsUnderDevelopmentAndDevelopableLandMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	182,061,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_ProjectsUnderDevelopmentAndDevelopableLandMember
Total	356,642,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_ProjectsUnderDevelopmentAndDevelopableLandMember
Accumulated depreciation	3,049,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = cli_ProjectsUnderDevelopmentAndDevelopableLandMember
Furniture, Fixtures And Equipment [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Costs Capitalized Subsequent to Acquisition	12,055,000us-gaap_RealEstateAndAccumulatedDepreciationCostsCapitalizedSubsequentToAcquisitionCarryingCosts / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_FurnitureAndFixturesMember
Gross Amount at Which Carried at Close of Period, Building and Improvements	12,055,000us-gaap_RealEstateAndAccumulatedDepreciationCarryingAmountOfBuildingsAndImprovements / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_FurnitureAndFixturesMember
Total	12,055,000us-gaap_RealEstateGrossAtCarryingValue / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_FurnitureAndFixturesMember
Accumulated depreciation	$ 3,515,000us-gaap_RealEstateAccumulatedDepreciation / us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis = us-gaap_FurnitureAndFixturesMember
Buildings And Improvements [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Useful Live	40 years
Minimum [Member] | Office [Member] | 9200 Edmonston Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1973
Minimum [Member] | Office [Member] | 6301 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1979
Minimum [Member] | Office [Member] | 6303 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1980
Minimum [Member] | Office [Member] | 6305 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1982
Minimum [Member] | Office [Member] | 6411 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1984
Maximum [Member] | Office [Member] | 9200 Edmonston Road [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2003
Maximum [Member] | Office [Member] | 6301 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1995
Maximum [Member] | Office [Member] | 6303 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2003
Maximum [Member] | Office [Member] | 6305 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	1995
Maximum [Member] | Office [Member] | 6411 Ivy Lane [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Year Built	2005
[1] The aggregate cost for federal income tax purposes at December 31, 2014 was approximately $3.2 billion. [2] Depreciation of buildings and improvements are calculated over lives ranging from the life of the lease to 40 years. [3] This property is located on land leased by the Company.