Annual report pursuant to Section 13 and 15(d)

Real Estate Transactions (Schedule Of Property Sales) (Details)

v2.4.1.9
Real Estate Transactions (Schedule Of Property Sales) (Details) (USD $)
3 Months Ended 12 Months Ended 0 Months Ended
Sep. 30, 2014
Jun. 30, 2014
Dec. 31, 2013
sqft
Sep. 30, 2013
Jun. 30, 2013
Dec. 31, 2014
item
sqft
Dec. 31, 2013
property
sqft
Dec. 31, 2012
property
Apr. 10, 2013
Apr. 26, 2013
Aug. 27, 2013
property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           16cli_NumberOfBuildingsOnPropertiesSold 24cli_NumberOfBuildingsOnPropertiesSold 3cli_NumberOfBuildingsOnPropertiesSold      
Rentable Square Feet     3,006,409cli_AreaOfRealEstatePropertySold     2,610,931cli_AreaOfRealEstatePropertySold 3,006,409cli_AreaOfRealEstatePropertySold        
Net Sales Proceeds           $ 319,239,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment $ 390,615,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment [1]        
Net Book Value     307,244,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment     264,391,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment 307,244,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment        
Realized Gain (loss)           54,848,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations 83,371,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations [2]        
Internal rate of return           9.00%cli_InternalRateOfReturn          
Ownership interest     11.90%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners     11.10%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners 11.90%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners        
Impairments     62,153,000us-gaap_AssetImpairmentCharges [3],[4] 48,700,000us-gaap_AssetImpairmentCharges [3]     110,853,000us-gaap_AssetImpairmentCharges 9,845,000us-gaap_AssetImpairmentCharges      
Deferred tax asset           14,400,000us-gaap_DeferredTaxAssetsValuationAllowance          
Note receivable, net present value     21,986,000us-gaap_LoansAndLeasesReceivableNetReportedAmount [5]     21,491,000us-gaap_LoansAndLeasesReceivableNetReportedAmount [5] 21,986,000us-gaap_LoansAndLeasesReceivableNetReportedAmount [5]        
Loss from early extinguishment of debt         (703,000)cli_DiscontinuedOperationGainsLossesOnExtinguishmentOfDebt   (703,000)cli_DiscontinuedOperationGainsLossesOnExtinguishmentOfDebt        
Number of impaired properties             18cli_NumberOfImpairedRealEstateProperties        
Number of land parcels sold             3cli_NumberOfLandParcelsSold        
Closing costs           26,136,000cli_CostsOfRealEstateServices 22,716,000cli_CostsOfRealEstateServices 3,746,000cli_CostsOfRealEstateServices      
Closing prorations and related adjustments             535,000cli_ProceedsFomSaleOfRealEstateHeldForInvestmentClosingProrationsAndRelatedAdjustments        
Realized gains (losses) on disposition of rental property, net 264,000us-gaap_GainLossOnDispositionOfAssets 54,584,000us-gaap_GainLossOnDispositionOfAssets       54,848,000us-gaap_GainLossOnDispositionOfAssets          
22 Sylvan Way [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwentyTwoSylvanWayMember
         
Rentable Square Feet           249,409cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwentyTwoSylvanWayMember
         
Net Sales Proceeds           94,897,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwentyTwoSylvanWayMember
         
Net Book Value           60,244,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwentyTwoSylvanWayMember
         
Realized Gain (loss)           34,653,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwentyTwoSylvanWayMember
         
30 Knightsbridge Road [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           4cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_ThirtyKnightsbridgeRoadMember
         
Rentable Square Feet           680,350cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_ThirtyKnightsbridgeRoadMember
         
Net Sales Proceeds           54,641,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_ThirtyKnightsbridgeRoadMember
         
Net Book Value           52,361,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_ThirtyKnightsbridgeRoadMember
         
Realized Gain (loss)           2,280,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_ThirtyKnightsbridgeRoadMember
         
470 Chestnut Ridge Road [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredSeventyChestnutRidgeRoadMember
         
Rentable Square Feet           52,500cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredSeventyChestnutRidgeRoadMember
         
Net Sales Proceeds           7,195,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredSeventyChestnutRidgeRoadMember
         
Net Book Value           7,109,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredSeventyChestnutRidgeRoadMember
         
Realized Gain (loss)           86,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredSeventyChestnutRidgeRoadMember
         
530 Chestnut Ridge Road [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredThirtyChestnutRidgeRoadMember
[6],[7]          
Rentable Square Feet           57,204cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredThirtyChestnutRidgeRoadMember
[6],[7]          
Net Sales Proceeds           6,299,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredThirtyChestnutRidgeRoadMember
[6],[7]          
Net Book Value           6,235,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredThirtyChestnutRidgeRoadMember
[6],[7]          
Realized Gain (loss)           64,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredThirtyChestnutRidgeRoadMember
[6],[7]          
400 Rella Boulevard [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredRellaBoulevardMember
         
Rentable Square Feet           180,000cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredRellaBoulevardMember
         
Net Sales Proceeds           27,539,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredRellaBoulevardMember
         
Net Book Value           10,938,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredRellaBoulevardMember
         
Realized Gain (loss)           16,601,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredRellaBoulevardMember
         
412 Mt. Kemble Avenue [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredTwelveMountKembleAvenueMember
[6]          
Rentable Square Feet           475,100cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredTwelveMountKembleAvenueMember
[6]          
Net Sales Proceeds           44,751,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredTwelveMountKembleAvenueMember
[6]          
Net Book Value           43,851,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredTwelveMountKembleAvenueMember
[6]          
Realized Gain (loss)           900,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FourHundredTwelveMountKembleAvenueMember
[6]          
17-17 Rte 208 North [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SeventeenDashSeventeenRouteTwoZeroEightNorthMember
[6],[7]          
Rentable Square Feet           143,000cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SeventeenDashSeventeenRouteTwoZeroEightNorthMember
[6],[7]          
Net Sales Proceeds           11,835,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SeventeenDashSeventeenRouteTwoZeroEightNorthMember
[6],[7]          
Net Book Value           11,731,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SeventeenDashSeventeenRouteTwoZeroEightNorthMember
[6],[7]          
Realized Gain (loss)           104,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SeventeenDashSeventeenRouteTwoZeroEightNorthMember
[6],[7]          
555, 565 And 570 Seventy Taxter Road [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           3cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredFiftyFiveFiveHundredSixtyFiveAndFiveHundredSeventyTaxterRoadMember
[6]          
Rentable Square Feet           416,108cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredFiftyFiveFiveHundredSixtyFiveAndFiveHundredSeventyTaxterRoadMember
[6]          
Net Sales Proceeds           41,057,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredFiftyFiveFiveHundredSixtyFiveAndFiveHundredSeventyTaxterRoadMember
[6]          
Net Book Value           41,057,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiveHundredFiftyFiveFiveHundredSixtyFiveAndFiveHundredSeventyTaxterRoadMember
[6]          
200 And 220 White Plains Road [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           2cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredAndTwoHundredTwentyWhitePlainsRoadMember
         
Rentable Square Feet           178,000cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredAndTwoHundredTwentyWhitePlainsRoadMember
         
Net Sales Proceeds           12,619,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredAndTwoHundredTwentyWhitePlainsRoadMember
         
Net Book Value           12,619,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredAndTwoHundredTwentyWhitePlainsRoadMember
         
1266 East Main Street [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings           1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneThousandTwoHundredSixtySixEastMainStreetMember
[6],[7]          
Rentable Square Feet           179,260cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneThousandTwoHundredSixtySixEastMainStreetMember
[6],[7]          
Net Sales Proceeds           18,406,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneThousandTwoHundredSixtySixEastMainStreetMember
[6],[7]          
Net Book Value           18,246,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneThousandTwoHundredSixtySixEastMainStreetMember
[6],[7]          
Realized Gain (loss)           160,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneThousandTwoHundredSixtySixEastMainStreetMember
[6],[7]          
Keystone Portfolio [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Net Sales Proceeds           196,800,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
         
Sale price of property           221,000,000cli_DiscontinuedOperationRealEstateSalePrice
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
         
Sale price of property, cash           192,500,000cli_DiscontinuedOperationRealEstateSalePriceCash
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
         
Sale price of property, capital accounts           21,200,000cli_DiscontinuedOperationRealEstateSalePriceCapitalAccounts
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
         
Closing costs paid           3,000,000cli_PaymentOfClosingCosts
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
         
Ownership interest           0.00%us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
         
Number of properties with senior pari passu interest           5cli_DiscontinuedOperationRealEstateSalePriceNumberOfPropertiesWithSeniorInterest
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
         
Sale price of property, capital balance in properties in which senior pari passu interest is held           7,300,000cli_DiscontinuedOperationRealEstateSalePricePartnersCapitalInPropertiesInWhichSeniorInterestIsHeld
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
         
Impairments             20,800,000us-gaap_AssetImpairmentCharges
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
       
Keystone Portfolio [Member] | Keystone Entities [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Internal rate of return           15.00%cli_InternalRateOfReturn
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
/ dei_LegalEntityAxis
= cli_KeystoneEntitiesMember
         
Keystone Portfolio [Member] | Parent Company [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Internal rate of return           10.00%cli_InternalRateOfReturn
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_KeystonePortfolioMember
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
         
19 Skyline Drive [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
[8]        
Rentable Square Feet     248,400cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
[8]       248,400cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
[8]        
Net Sales Proceeds             16,131,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
[8]   16,100,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
   
Net Book Value     16,005,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
[8]       16,005,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
[8]        
Realized Gain (loss)             126,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
[8]   100,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
   
Deferred tax asset               7,100,000us-gaap_DeferredTaxAssetsValuationAllowance
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
     
Note receivable     5,000,000us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
      5,000,000us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
       
Note receivable, net present value     3,700,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
      3,700,000us-gaap_LoansAndLeasesReceivableNetReportedAmount
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_NineteenSkylineDriveMember
       
55 Corporate Drive [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
       
Rentable Square Feet     204,057cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
      204,057cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
       
Net Sales Proceeds             70,967,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
    71,000,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
 
Net Book Value     51,308,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
      51,308,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
60,900,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
     
Realized Gain (loss)             19,659,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
    19,700,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
 
Number of properties held for sale               2cli_NumberOfRealEstatePropertiesHeldForSale
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
     
Deferred tax asset               7,100,000us-gaap_DeferredTaxAssetsValuationAllowance
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyFiveCorporateDriveMember
     
200 Riser Road [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredRiserRoadMember
       
Rentable Square Feet     286,628cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredRiserRoadMember
      286,628cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredRiserRoadMember
       
Net Sales Proceeds             31,775,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredRiserRoadMember
       
Net Book Value     14,852,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredRiserRoadMember
      14,852,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredRiserRoadMember
       
Realized Gain (loss)             16,923,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_TwoHundredRiserRoadMember
       
777 Passaic Avenue [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SevenHundredSeventySevenPassaicAvenueMember
       
Rentable Square Feet     75,000cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SevenHundredSeventySevenPassaicAvenueMember
      75,000cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SevenHundredSeventySevenPassaicAvenueMember
       
Net Sales Proceeds             5,640,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SevenHundredSeventySevenPassaicAvenueMember
       
Net Book Value     3,713,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SevenHundredSeventySevenPassaicAvenueMember
      3,713,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SevenHundredSeventySevenPassaicAvenueMember
       
Realized Gain (loss)             1,927,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SevenHundredSeventySevenPassaicAvenueMember
       
16 And 18 Sentry Park West [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             2cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SixteenAndEighteenSentryParkWestMember
[9]        
Rentable Square Feet     188,103cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SixteenAndEighteenSentryParkWestMember
[9]       188,103cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SixteenAndEighteenSentryParkWestMember
[9]        
Net Sales Proceeds             19,041,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SixteenAndEighteenSentryParkWestMember
[9]        
Net Book Value     19,721,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SixteenAndEighteenSentryParkWestMember
[9]       19,721,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SixteenAndEighteenSentryParkWestMember
[9]        
Realized Gain (loss)             (680,000)us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SixteenAndEighteenSentryParkWestMember
[9]        
Impairments               8,400,000us-gaap_AssetImpairmentCharges
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_SixteenAndEighteenSentryParkWestMember
     
51 Imclone Drive [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyOneImcloneDriveMember
[10]        
Rentable Square Feet     63,213cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyOneImcloneDriveMember
[10]       63,213cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyOneImcloneDriveMember
[10]        
Net Sales Proceeds             6,101,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyOneImcloneDriveMember
[10]        
Net Book Value     5,278,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyOneImcloneDriveMember
[10]       5,278,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyOneImcloneDriveMember
[10]        
Realized Gain (loss)             823,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FiftyOneImcloneDriveMember
[10]        
40 Richards Avenue [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FortyRichardsAvenueMember
       
Rentable Square Feet     145,487cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FortyRichardsAvenueMember
      145,487cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FortyRichardsAvenueMember
       
Net Sales Proceeds             15,858,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FortyRichardsAvenueMember
       
Net Book Value     17,027,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FortyRichardsAvenueMember
      17,027,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FortyRichardsAvenueMember
       
Realized Gain (loss)             (1,169,000)us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_FortyRichardsAvenueMember
       
106 Allen Road [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             1cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneHunderedSixAllenRoadMember
       
Rentable Square Feet     132,010cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneHunderedSixAllenRoadMember
      132,010cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneHunderedSixAllenRoadMember
       
Net Sales Proceeds             17,677,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneHunderedSixAllenRoadMember
       
Net Book Value     15,081,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneHunderedSixAllenRoadMember
      15,081,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneHunderedSixAllenRoadMember
       
Realized Gain (loss)             2,596,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_OneHunderedSixAllenRoadMember
       
Pennsylvania Office Portfolio [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Number of Buildings             15cli_NumberOfBuildingsOnPropertiesSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
[11],[12]        
Rentable Square Feet     1,663,511cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
[11],[12]       1,663,511cli_AreaOfRealEstatePropertySold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
[11],[12]        
Net Sales Proceeds             207,425,000us-gaap_ProceedsFromSaleOfRealEstateHeldforinvestment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
[11],[12]        
Net Book Value     164,259,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
[11],[12]       164,259,000us-gaap_DisposalGroupIncludingDiscontinuedOperationPropertyPlantAndEquipment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
[11],[12]        
Realized Gain (loss)             43,166,000us-gaap_GainLossOnDispositionOfRealEstateDiscontinuedOperations
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
[11],[12]        
Sale price of property                     233,000,000cli_DiscontinuedOperationRealEstateSalePrice
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Sale price of property, cash                     201,000,000cli_DiscontinuedOperationRealEstateSalePriceCash
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Internal rate of return                     10.00%cli_InternalRateOfReturn
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Impairments             23,851,000us-gaap_AssetImpairmentCharges
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
       
Number of impaired properties             5cli_NumberOfImpairedRealEstateProperties
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
       
Number of land parcels sold                     3cli_NumberOfLandParcelsSold
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Amount held by intermediary for reinvestment     55,300,000cli_ProceedsFromSaleOfRealEstateAmountHeldByIntermediaryForReinvesment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
      55,300,000cli_ProceedsFromSaleOfRealEstateAmountHeldByIntermediaryForReinvesment
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
       
New mortgage loan                     10,000,000us-gaap_MortgageLoansOnRealEstateNewMortgageLoans
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Number of mortgage loans                     1us-gaap_MortgageLoansOnRealEstateNumberOfLoans
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Capital balance                     22,000,000us-gaap_PartnersCapital
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Closing costs                     4,000,000cli_CostsOfRealEstateServices
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Mortgage loan, maturity period                     2 years
Loan extension period                     1 year
Spread over LIBOR                     6.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Amount of mortgage funded                     $ 8,000,000cli_MortgageLoansOnRealEstateAmountOfMortgageFundedAtClosing
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
Pennsylvania Office Portfolio [Member] | Keystone Entities [Member]                      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                      
Initial internal rate of return                     15.00%cli_InitialInternalRateOfReturn
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
/ dei_LegalEntityAxis
= cli_KeystoneEntitiesMember
Third party ownership percentage     0.00%cli_ThirdPartyOwnershipPercent
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
/ dei_LegalEntityAxis
= cli_KeystoneEntitiesMember
      0.00%cli_ThirdPartyOwnershipPercent
/ us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis
= cli_PennsylvaniaOfficePortfolioMember
/ dei_LegalEntityAxis
= cli_KeystoneEntitiesMember
       
[1] This amount excludes approximately $535,000 of net closing prorations and related adjustments received from sellers at closing.
[2] This amount, net of impairment charges recorded in 2013 of $23,851,000 on certain of the properties prior to their sale (per Note [d] above), comprises the $59,520,000 of realized gains (losses) and unrealized losses on disposition of rental property and impairments, net, for the year ended December 31, 2013.  See Note 7: Discontinued Operations.
[3] Amounts for the year ended December 31, 2013 relate to impairment charges as further described in Note 3: Real Estate Transactions - Impairments on Properties Held and Used.
[4] During the quarter ended December 31, 2013, the Company identified and recorded an out-of-period adjustment to reflect a charge of $1,260,000 to correct an error in the measurement of its impairment charges on certain properties in the third quarter of 2013. In the third quarter, in measuring the impairments on these properties, the Company did not include certain tenant improvement amounts to be paid for in-place leases in its discounted cash flows used to measure fair value and, as a result, should have recognized larger impairment charges by this aggregate amount. The Company has determined that this adjustment was not material to the quarter ended December 31, 2013 or the prior interim period.
[5] Includes as of December 31, 2014: a mortgage receivable for $10.4 million which bears interest at LIBOR plus six percent and matures in August 2015; a note receivable for $7.8 million which bears interest at eight percent and matures in October 2017; and an interest-free note receivable with a net present value of $3.3 million and matures in April 2023. The Company believes these balances are fully collectable.
[6] The Company completed the sale of these properties for approximately $221 million, comprised of: $192.5 million in cash from a combination of affiliates of Keystone Property Group’s (“Keystone Entities”) senior and pari-passu equity and mortgage financing; Company subordinated equity interests in each of the properties sold with capital accounts aggregating $21.2 million; and Company pari-passu equity interests in five of the properties sold aggregating $7.3 million. Net sale proceeds from the sale aggregated $196.8 million which was comprised of the $221 million gross sales price less the subordinated equity interests of $21.2 million and $3 million in closing costs. The purchasers of these properties are unconsolidated joint ventures formed between the Company and the Keystone Entities. The senior and pari-passu equity will receive a 15 percent internal rate of return (“IRR”) after which the subordinated equity will receive a 10 percent IRR and then all distributable cash flow will be split equally between the Keystone Entities and the Company. See Note 4: Investments in Unconsolidated Joint Ventures. In connection with certain of these partial sale transactions, because the buyer received a preferential return on certain of the ventures for which the Company received subordinated equity interests, the Company only recognized profit to the extent that they received net proceeds in excess of their entire carrying value of the properties, effectively reflecting their retained subordinated equity interest at zero.
[7] The Company recorded an impairment charge of $20.8 million on these properties at December 31, 2013 as it estimated that the carrying value of the properties may not be recoverable over their anticipated holding periods.
[8] The Company recognized a valuation allowance of $7.1 million on this property identified as held for sale at December 31, 2012. In connection with the sale, the Company provided an interest-free note receivable to the buyer of $5 million (with a net present value of $3.7 million at closing) which matures in 2023 and requires monthly payments of principal. See Note 5: Deferred charges, goodwill and other assets.
[9] The Company recorded an $8.4 million impairment charge on these properties at December 31, 2012. The Company has retained a subordinated interest in these properties.
[10] The property was encumbered by a mortgage loan which was satisfied by the Company at the time of the sale. The Company incurred $0.7 million in costs for the debt satisfaction, which was included in discontinued operations: loss from early extinguishment of debt for the year ended December 31, 2013.
[11] In order to reduce the carrying value of five of the properties to their estimated fair market values, the Company recorded impairment charges of $23,851,000 at June 30, 2013. The fair value used in the impairment charges was based on the purchase and sale agreement for the properties ultimately sold.
[12] The Company completed the sale of this office portfolio and three developable land parcels for approximately $233 million: $201 million in cash ($55.3 million of which was held by a qualified intermediary until such funds were used in acquisitions), a $10 million mortgage on one of the properties ($8 million of which was funded at closing) and subordinated equity interests in each of the properties being sold with capital accounts aggregating $22 million. Net sale proceeds from the sale aggregated $207 million which was comprised of the $233 million gross sales price less the subordinated equity interests of $22 million and $4 million in closing costs. The purchasers of the Pennsylvania office portfolio are joint ventures formed between the Company and affiliates of the Keystone Property Group (the “Keystone Affiliates”). The mortgage loan has a term of two years with a one year extension option and bears interest at LIBOR plus six percent. The Company's equity interests in the joint ventures will be subordinated to Keystone Affiliates receiving a 15 percent internal rate of return (“IRR”) after which the Company will receive a ten percent IRR on its subordinated equity and then all profit will be split equally. In connection with these partial sale transactions, because the buyer receives a preferential return, the Company only recognized profit to the extent that they received net proceeds in excess of their entire carrying value of the properties, effectively reflecting their retained subordinate equity interest at zero. As part of the transaction, the Company has rights to own, after zoning-approval-subdivision, land at the 150 Monument Road property located in Bala Cynwyd, Pennsylvania, for a contemplated multi-family residential development.