Annual report pursuant to Section 13 and 15(d)

Investments In Unconsolidated Joint Ventures (Harborside Unit A Urban Renewal, L.L.C.) (Narrative) (Details)

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Investments In Unconsolidated Joint Ventures (Harborside Unit A Urban Renewal, L.L.C.) (Narrative) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
May 21, 2014
Aug. 01, 2014
Investments in unconsolidated joint ventures $ 247,468,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures $ 181,129,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures      
Internal rate of return 9.00%cli_InternalRateOfReturn        
Development of rental property 25,140,000us-gaap_PaymentsToDevelopRealEstateAssets 13,772,000us-gaap_PaymentsToDevelopRealEstateAssets 60,354,000us-gaap_PaymentsToDevelopRealEstateAssets    
Parent Company [Member]          
Percentage of capital event cash flows distributed 50.00%cli_PercentageOfCapitalEventCashFlowsDistributed
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember 		       
Harborside Unit A Urban Renewal, LLC [Member]          
Number of units 763us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Interest rate of senior unsecured notes 5.197%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		[1]        
Percentage of interest in venture 85.00%us-gaap_MinorityInterestOwnershipPercentageByParent
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Investments in unconsolidated joint ventures 34,954,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		    30,600,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		 
Capital credit receivable per developable square foot 30cli_CapitalCreditReceivablePerDevelopableSquareFoot
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Holding and distribution pattern under operating agreement In general, the operating agreement of URL-Harborside provides that net operating cash flows are distributed first, to the members in respect of preferred return, as defined, until each member shall have received payment of the accrued and unpaid preferred return; and, thereafter, 75 percent to the Company and 25 percent to ISA.        
Development of rental property 19,200,000us-gaap_PaymentsToDevelopRealEstateAssets
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Harborside Unit A Urban Renewal, LLC [Member] | Construction Loan [Member]          
Maximum borrowing capacity 192,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		      192,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
Amount outstanding 0us-gaap_LineOfCredit
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Interest rate of senior unsecured notes         5.197%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
Maturity date of the senior unsecured notes Aug. 01, 2029        
Harborside Unit A Urban Renewal, LLC [Member] | Ironstate Harborside-A, LLC [Member]          
Investments in unconsolidated joint ventures 5,400,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ dei_LegalEntityAxis
= cli_IronstateHarborsideaLLCMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Percentage of capital event cash flows distributed after initial internal rate of return reached 35.00%cli_PercentageOfCapitalEventCashFlowsDistributedAfterInitialInternalRateOfReturnReached
/ dei_LegalEntityAxis
= cli_IronstateHarborsideaLLCMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Percentage of capital event cash flows distributed 25.00%cli_PercentageOfCapitalEventCashFlowsDistributed
/ dei_LegalEntityAxis
= cli_IronstateHarborsideaLLCMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Percentage of operating return on capital 25.00%cli_PercentageOfOperatingReturnOnCapital
/ dei_LegalEntityAxis
= cli_IronstateHarborsideaLLCMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Harborside Unit A Urban Renewal, LLC [Member] | Parent Company [Member]          
Percentage of capital event cash flows distributed after initial internal rate of return reached 65.00%cli_PercentageOfCapitalEventCashFlowsDistributedAfterInitialInternalRateOfReturnReached
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Percentage of capital event cash flows distributed 75.00%cli_PercentageOfCapitalEventCashFlowsDistributed
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Percentage of operating return on capital 75.00%cli_PercentageOfOperatingReturnOnCapital
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Internal rate of return 18.00%cli_InternalRateOfReturn
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Harborside Unit A Urban Renewal, LLC [Member] | Land [Member] | Parent Company [Member]          
Investments in unconsolidated joint ventures 5,500,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_LandMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
KPG-MCG Curtis JV, LLC [Member]          
Investments in unconsolidated joint ventures 59,911,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgMcgCurtisJvLLCMember 		[2]        
Aggregate capital credits 20,600,000cli_AggregateCapitalCredits
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgMcgCurtisJvLLCMember 		       
Holding and distribution pattern under operating agreement In general, the operating agreement of the Curtis Center JV provides that net cash flows from operations and capital events are distributed first, to the members, pro rata in proportion to their unreturned capital contributions, until each member's unreturned capital contributions have been reduced to zero; and, thereafter, to each member, pro rata, in accordance with their percentage interests.        
Harborside Residential Project [Member]          
Development Costs, Period Cost 65,100,000us-gaap_DevelopmentCosts
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_HarborsideResidentialProjectMember 		       
Harborside Residential Project [Member] | Harborside Unit A Urban Renewal, LLC [Member]          
Number of units 763us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_HarborsideResidentialProjectMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Total project costs 320,000,000cli_EstimatedCostOfProject
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_HarborsideResidentialProjectMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
State tax credit 33,000,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwards
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_HarborsideResidentialProjectMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
Harborside Residential Project [Member] | Harborside Unit A Urban Renewal, LLC [Member] | Parent Company [Member]          
Total project costs $ 59,100,000cli_EstimatedCostOfProject
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_HarborsideResidentialProjectMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember 		       
[1] The construction/permanent loan has a maximum borrowing amount of $192,000.
[2] Includes undivided interests in the same manner as investments in noncontrolling partnership, pursuant to ASC 970-323-25-12. See discussion in Unconsolidated Joint Venture Transactions following in this footnote.