Quarterly report pursuant to Section 13 or 15(d)

Investments In Unconsolidated Joint Ventures (Summary Of Unconsolidated Joint Ventures) (Details)

v2.4.1.9
Investments In Unconsolidated Joint Ventures (Summary Of Unconsolidated Joint Ventures) (Details) (USD $)
3 Months Ended 0 Months Ended
Mar. 31, 2015
item
Sep. 02, 2013
Jun. 30, 2013
Aug. 01, 2014
Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]          
Carrying Amount $ 262,052,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures       $ 247,468,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
Property Debt, Balance 1,787,191,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages        
Amount outstanding 42,000,000us-gaap_LineOfCredit       0us-gaap_LineOfCredit
Accounts payable, accrued expenses and other liabilities 134,462,000us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent       126,971,000us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent
Minimum [Member]          
Schedule of Equity Method Investments [Line Items]          
Company's Effective Ownership % 7.50%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
       
Maximum [Member]          
Schedule of Equity Method Investments [Line Items]          
Company's Effective Ownership % 85.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
       
Marbella RoseGarden, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 412us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_MarbellaRoseGardenLLCMember
[1]        
Company's Effective Ownership % 24.27%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_MarbellaRoseGardenLLCMember
[1],[2]        
Carrying Amount 15,839,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_MarbellaRoseGardenLLCMember
[1]       15,779,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_MarbellaRoseGardenLLCMember
[1]
Property Debt, Balance 95,000,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_MarbellaRoseGardenLLCMember
[1]        
Property Debt, Maturity Date 05/01/18 [1]        
Property Debt, Interest Rate 4.99%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_MarbellaRoseGardenLLCMember
[1]        
RoseGarden Monaco Holdings, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 523us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoHoldingsLLCMember
[1]        
Company's Effective Ownership % 15.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoHoldingsLLCMember
[1],[2]        
Carrying Amount 1,845,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoHoldingsLLCMember
[1]       2,161,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoHoldingsLLCMember
[1]
Property Debt, Balance 165,000,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoHoldingsLLCMember
[1]        
Property Debt, Maturity Date 02/01/21 [1]        
Property Debt, Interest Rate 4.19%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoHoldingsLLCMember
[1]        
Rosewood Lafayette Holdings, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 217us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodLafayetteHoldingsLLCMember
[1]        
Company's Effective Ownership % 25.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodLafayetteHoldingsLLCMember
[1],[2]        
Carrying Amount         62,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodLafayetteHoldingsLLCMember
[1]
Property Debt, Balance 38,478,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodLafayetteHoldingsLLCMember
[1]        
Property Debt, Maturity Date 07/01/15 [1]        
Property Debt, Interest Rate 4.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodLafayetteHoldingsLLCMember
[1]        
PruRose Port Imperial South 15, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 236us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthFifteenLLCMember
[1]        
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthFifteenLLCMember
[1],[2]        
Property Debt, Balance 57,500,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthFifteenLLCMember
[1]        
Property Debt, Maturity Date 09/01/20 [1]        
Property Debt, Interest Rate 4.32%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthFifteenLLCMember
[1]        
Rosewood Morristown, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 130us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
[3],[4]        
Company's Effective Ownership % 12.50%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
[2],[3],[4]        
Carrying Amount 5,920,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
[3],[4]       6,029,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
[3],[4]
Property Debt, Balance 46,217,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
[3],[4]        
Property Debt, Maturity Date (e) [3],[4]        
Property Debt, Interest Rate    [3],[4],[5]        
Overlook Ridge JV, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 251us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJVLLCMember
[1]        
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJVLLCMember
[1],[2]        
Property Debt, Balance 69,420,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJVLLCMember
[1]        
Property Debt, Maturity Date (f) [1]        
Property Debt, Interest Rate    [1],[6]        
Overlook Ridge JV 2C/3B, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 371us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
[1]        
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
[1],[2]        
Carrying Amount 2,304,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
[1]       2,524,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
[1]
Property Debt, Balance 51,447,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
[1]        
Property Debt, Maturity Date 12/26/15 [1]        
Property Debt, Interest Rate, LIBOR L+2.50 [1],[7]        
Property Debt, Interest Rate, Spread Over LIBOR 2.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
[1],[7]        
Notional amount 52,000,000us-gaap_NotionalAmountOfNonderivativeInstruments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
       
Overlook Ridge JV 2C/3B, L.L.C. [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Interest Rate 3.0875%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
       
Number of extension options 2cli_DebtInstrumentNumberOfExtensionOptions
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
       
Loan extension period 1 year        
Maximum borrowing capacity 55,500,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
       
Notional amount   1,840,000us-gaap_NotionalAmountOfNonderivativeInstruments
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
     
Overlook Ridge JV 2C/3B, L.L.C. [Member] | Construction Loan Extension Number 1 [Member]          
Schedule of Equity Method Investments [Line Items]          
Extension fee 0.25%cli_DebtInstrumentExtensionFeePercentage
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanExtensionNumberOneMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJvTwoCAndThreeBLLCMember
       
PruRose Riverwalk G, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 316us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRoseRiverwalkGLLCMember
[1]        
Company's Effective Ownership % 25.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRoseRiverwalkGLLCMember
[1],[2]        
Carrying Amount 701,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRoseRiverwalkGLLCMember
[1]       955,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRoseRiverwalkGLLCMember
[1]
Property Debt, Balance 79,154,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRoseRiverwalkGLLCMember
[1]        
Property Debt, Maturity Date 07/15/21 [1]        
Property Debt, Interest Rate 6.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRoseRiverwalkGLLCMember
[1],[8]        
Maximum borrowing capacity 80,249,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRoseRiverwalkGLLCMember
       
Elmajo Urban Renewal Associates, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 355us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_ElmajoUrbanRenewalAssociatesLLCMember
[1]        
Company's Effective Ownership % 7.50%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_ElmajoUrbanRenewalAssociatesLLCMember
[1],[2]        
Property Debt, Balance 128,100,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_ElmajoUrbanRenewalAssociatesLLCMember
[1]        
Property Debt, Maturity Date 03/01/2030 [1]        
Property Debt, Interest Rate 4.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_ElmajoUrbanRenewalAssociatesLLCMember
[1],[9]        
Elmajo Urban Renewal Associates, L.L.C. [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Maximum borrowing capacity 91,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_ElmajoUrbanRenewalAssociatesLLCMember
       
Crystal House Apartments Investors LLC [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 828us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
[10]        
Company's Effective Ownership % 25.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
[10],[2]        
Carrying Amount 27,340,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
[10]       27,051,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
[10]
Property Debt, Balance 165,000,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
[10]        
Property Debt, Maturity Date 04/01/20 [10]        
Property Debt, Interest Rate 3.17%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
[10]        
Percentage of interest in developable land 50.00%cli_PercentageOfInterestInDevelopableLand
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
       
Number of units available for development 295cli_NumberOfUnitsAvailableForDevelopment
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
       
Number of approved units available for development 252cli_NumberOfApprovedUnitsAvailableForDevelopment
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CrystalHouseApartmentsInvestorsLLCMember
       
Portside Master Company, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 176us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
[1]        
Company's Effective Ownership % 38.25%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
[1],[2]        
Carrying Amount 1,017,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
[1]       1,747,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
[1]
Property Debt, Balance 38,990,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
[1]        
Property Debt, Maturity Date 12/04/15 [1]        
Property Debt, Interest Rate, LIBOR L+2.50 [1],[11]        
Property Debt, Interest Rate, Spread Over LIBOR 2.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
[1],[11]        
Portside Master Company, L.L.C. [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of extension options 2cli_DebtInstrumentNumberOfExtensionOptions
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
       
Maximum borrowing capacity 42,500,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
       
Portside Master Company, L.L.C. [Member] | Construction Loan Extension Number 1 [Member]          
Schedule of Equity Method Investments [Line Items]          
Loan extension period 2 years        
Extension fee 0.125%cli_DebtInstrumentExtensionFeePercentage
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanExtensionNumberOneMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
       
Portside Master Company, L.L.C. [Member] | Construction Loan Extension Number 2 [Member]          
Schedule of Equity Method Investments [Line Items]          
Loan extension period 2 years        
Extension fee 0.25%cli_DebtInstrumentExtensionFeePercentage
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanExtensionNumberTwoMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PortsideMasterCompanyLLCMember
       
PruRose Port Imperial South 13, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 280us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
[1]        
Company's Effective Ownership % 20.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
[1],[2]        
Carrying Amount 815,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
[1]       1,087,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
[1]
Property Debt, Balance 56,999,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
[1]        
Property Debt, Maturity Date 06/27/16 [1]        
Property Debt, Interest Rate, LIBOR L+2.15 [1],[12]        
Property Debt, Interest Rate, Spread Over LIBOR 2.15%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
[1],[12]        
PruRose Port Imperial South 13, L.L.C. [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Interest Rate 2.79%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
       
Maximum borrowing capacity 73,350,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
       
Notional amount 69,500,000us-gaap_NotionalAmountOfNonderivativeInstruments
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
  1,620,000us-gaap_NotionalAmountOfNonderivativeInstruments
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
   
PruRose Port Imperial South 13, L.L.C. [Member] | Construction Loan Extension Number 1 [Member]          
Schedule of Equity Method Investments [Line Items]          
Loan extension period 1 year        
Extension fee 0.25%cli_DebtInstrumentExtensionFeePercentage
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanExtensionNumberOneMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PruRosePortImperialSouthThirteenLLCMember
       
PruRose Port Imperial South 13, L.L.C. [Member] | Construction Loan Extension Number 2 [Member]          
Schedule of Equity Method Investments [Line Items]          
Loan extension period 6 months        
Roseland/Port Imperial Partners, L.P. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 363us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandPortImperialPartnersLPMember
[1],[13]        
Company's Effective Ownership % 20.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandPortImperialPartnersLPMember
[1],[13],[2]        
Carrying Amount 1,755,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandPortImperialPartnersLPMember
[1],[13]       1,800,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandPortImperialPartnersLPMember
[1],[13]
Property Debt, Maturity Date - [1],[13]        
RoseGarden Marbella South, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 311us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
       
Company's Effective Ownership % 24.27%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
[2]        
Carrying Amount 12,920,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
      11,282,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
Property Debt, Balance 43,090,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
       
Property Debt, Maturity Date 03/30/17        
Property Debt, Interest Rate, LIBOR L+2.25 [14]        
Property Debt, Interest Rate, Spread Over LIBOR 2.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
[14]        
RoseGarden Marbella South, L.L.C. [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of extension options 2cli_DebtInstrumentNumberOfExtensionOptions
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
       
Loan extension period 1 year        
Extension fee 0.25%cli_DebtInstrumentExtensionFeePercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
       
Maximum borrowing capacity 77,400,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMarbellaSouthLLCMember
       
Estuary Urban Renewal Unit B, LLC [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 227us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_EstuaryUrbanRenewalUnitBLLCMember
[1]        
Company's Effective Ownership % 7.50%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_EstuaryUrbanRenewalUnitBLLCMember
[1],[2]        
Property Debt, Balance 81,900,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_EstuaryUrbanRenewalUnitBLLCMember
[1]        
Property Debt, Maturity Date 03/01/2030 [1]        
Property Debt, Interest Rate 4.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_EstuaryUrbanRenewalUnitBLLCMember
[1],[15]        
Estuary Urban Renewal Unit B, LLC [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Maximum borrowing capacity 57,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_EstuaryUrbanRenewalUnitBLLCMember
       
RiverPark At Harrison I, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 141us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
       
Company's Effective Ownership % 36.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
[2]        
Carrying Amount 4,571,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
      4,744,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
Property Debt, Balance 23,095,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
       
Property Debt, Maturity Date 06/27/16        
Property Debt, Interest Rate, LIBOR L+2.35 [16]        
Property Debt, Interest Rate, Spread Over LIBOR 2.35%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
[16]        
RiverPark At Harrison I, L.L.C. [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of extension options 2cli_DebtInstrumentNumberOfExtensionOptions
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
       
Loan extension period 1 year        
Maximum borrowing capacity 23,400,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
       
RiverPark At Harrison I, L.L.C. [Member] | Construction Loan Extension Number 1 [Member]          
Schedule of Equity Method Investments [Line Items]          
Extension fee 0.20%cli_DebtInstrumentExtensionFeePercentage
/ us-gaap_LongtermDebtTypeAxis
= cli_ConstructionLoanExtensionNumberOneMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RiverparkAtHarrisonILLCMember
       
Capitol Place Mezz LLC [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 377us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CapitolPlaceMezzLlcMember
       
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CapitolPlaceMezzLlcMember
[2]        
Carrying Amount 49,873,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CapitolPlaceMezzLlcMember
      49,327,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CapitolPlaceMezzLlcMember
Property Debt, Balance 73,971,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CapitolPlaceMezzLlcMember
       
Property Debt, Maturity Date 07/01/2033        
Property Debt, Interest Rate 4.82%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CapitolPlaceMezzLlcMember
[17]        
Capitol Place Mezz LLC [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Maximum borrowing capacity 100,700,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_CapitolPlaceMezzLlcMember
       
Harborside Unit A Urban Renewal, LLC [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 763us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
       
Company's Effective Ownership % 85.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
[2]        
Carrying Amount 52,263,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
      34,954,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
Property Debt, Maturity Date 08/01/29        
Property Debt, Interest Rate 5.197%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
[18]     5.197%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
 
Maximum borrowing capacity 192,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
       
Amount outstanding 0us-gaap_LineOfCredit
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
       
Harborside Unit A Urban Renewal, LLC [Member] | Construction Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Maximum borrowing capacity 192,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_ConstructionLoansMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HarborsideUnitaUrbanRenewalLLCMember
       
Maturity       Aug. 01, 2029  
RoseGarden Monaco, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 300us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoLLCMember
       
Company's Effective Ownership % 41.67%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoLLCMember
[2]        
Carrying Amount 1,283,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoLLCMember
      1,283,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoseGardenMonacoLLCMember
Property Debt, Maturity Date -        
Grand Jersey Waterfront Urban Renewal Associates, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 1,000us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_GrandJerseyWaterfrontUrbanRenewalAssociatesLLCMember
       
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_GrandJerseyWaterfrontUrbanRenewalAssociatesLLCMember
[2]        
Carrying Amount 337,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_GrandJerseyWaterfrontUrbanRenewalAssociatesLLCMember
      337,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_GrandJerseyWaterfrontUrbanRenewalAssociatesLLCMember
Property Debt, Maturity Date -        
Red Bank Corporate Plaza, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 92,878us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RedBankCorporatePlazaLLCMember
       
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RedBankCorporatePlazaLLCMember
[2]        
Carrying Amount 4,081,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RedBankCorporatePlazaLLCMember
      3,963,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RedBankCorporatePlazaLLCMember
Property Debt, Balance 15,682,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RedBankCorporatePlazaLLCMember
       
Property Debt, Maturity Date 05/17/16        
Property Debt, Interest Rate 3.99375%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RedBankCorporatePlazaLLCMember
       
Property Debt, Interest Rate, LIBOR L+3.00 [19]        
Property Debt, Interest Rate, Spread Over LIBOR 3.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RedBankCorporatePlazaLLCMember
[19]        
Notional amount 13,650,000us-gaap_NotionalAmountOfNonderivativeInstruments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RedBankCorporatePlazaLLCMember
       
12 Vreeland Associates, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 139,750us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_TwelveVreelandAssociatesLLCMember
       
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_TwelveVreelandAssociatesLLCMember
[2]        
Carrying Amount 5,606,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_TwelveVreelandAssociatesLLCMember
      5,620,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_TwelveVreelandAssociatesLLCMember
Property Debt, Balance 13,679,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_TwelveVreelandAssociatesLLCMember
       
Property Debt, Maturity Date 07/01/23        
Property Debt, Interest Rate 2.87%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_TwelveVreelandAssociatesLLCMember
       
BNES Associates III [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 106,345us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_BNESAssociatesIIIMember
       
Company's Effective Ownership % 31.25%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_BNESAssociatesIIIMember
[2]        
Carrying Amount 2,061,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_BNESAssociatesIIIMember
      1,993,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_BNESAssociatesIIIMember
Property Debt, Balance 6,603,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_BNESAssociatesIIIMember
       
Property Debt, Maturity Date 11/01/23        
Property Debt, Interest Rate 4.76%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_BNESAssociatesIIIMember
       
Hillsborough 206 Holdings, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 160,000us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HillsboroughTwoHundredSixHoldingsLLCMember
       
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HillsboroughTwoHundredSixHoldingsLLCMember
[2]        
Carrying Amount 1,962,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HillsboroughTwoHundredSixHoldingsLLCMember
      1,962,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_HillsboroughTwoHundredSixHoldingsLLCMember
Property Debt, Maturity Date -        
KPG-P 100 IMW JV, LLC [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 339,615us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgPOneHundredImwJvLlcMember
       
Company's Effective Ownership % 33.33%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgPOneHundredImwJvLlcMember
[2]        
Carrying Amount 416,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgPOneHundredImwJvLlcMember
       
Property Debt, Balance 61,500,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgPOneHundredImwJvLlcMember
       
Property Debt, Maturity Date 09/09/16        
Property Debt, Interest Rate, LIBOR L+7.00 [20]        
Property Debt, Interest Rate, Spread Over LIBOR 7.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgPOneHundredImwJvLlcMember
[20]        
Number of extension options 2cli_DebtInstrumentNumberOfExtensionOptions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgPOneHundredImwJvLlcMember
       
Loan extension period 1 year        
Amount outstanding 13,600,000us-gaap_LineOfCredit
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgPOneHundredImwJvLlcMember
       
Construction reserve 25,000,000cli_LineOfCreditFacilityConstructionReserve
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgPOneHundredImwJvLlcMember
       
Keystone-Penn [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 1,842,820us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Company's Effective Ownership %    [2],[20],[21]        
Property Debt, Balance 205,946,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Property Debt, Maturity Date (v)        
Property Debt, Interest Rate    [22]        
Keystone-Penn [Member] | Principal Balance Due August 27, 2023 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 127,600,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustTwoThousandTwentyThreeMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Property Debt, Maturity Date August 27, 2023        
Property Debt, Interest Rate 5.114%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustTwoThousandTwentyThreeMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Keystone-Penn [Member] | Principal Balance Due August 27, 2016 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 67,921,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustTwoThousandSixteenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Property Debt, Maturity Date August 27, 2016        
Keystone-Penn [Member] | Principal Balance Due August 27, 2016 [Member] | Minimum [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Interest Rate, Spread Over LIBOR 5.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustTwoThousandSixteenMember
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Keystone-Penn [Member] | Principal Balance Due August 27, 2016 [Member] | Maximum [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Interest Rate, Spread Over LIBOR 5.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustTwoThousandSixteenMember
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Keystone-Penn [Member] | Principal Balance Due August 27, 2015 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 10,425,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustTwoThousandFifteenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Property Debt, Maturity Date August 27, 2015        
Property Debt, Interest Rate, Spread Over LIBOR 6.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustTwoThousandFifteenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Keystone-Penn [Member] | Keystone Property Group [Member]          
Schedule of Equity Method Investments [Line Items]          
Internal rate of return 15.00%cli_InternalRateOfReturn
/ dei_LegalEntityAxis
= cli_KeystonePropertyGroupMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Keystone-Penn [Member] | Parent Company [Member]          
Schedule of Equity Method Investments [Line Items]          
Internal rate of return 10.00%cli_InternalRateOfReturn
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystonePennMember
       
Keystone-TriState [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 1,266,384us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Company's Effective Ownership %    [2],[23]        
Carrying Amount 4,792,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
      6,140,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
Property Debt, Balance 205,112,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Property Debt, Maturity Date (x)        
Property Debt, Interest Rate    [24]        
Capital balance in properties in which senior pari passu interest is held 4,800,000cli_RealEstateSalePricePartnersCapitalInPropertiesInWhichSeniorInterestIsHeld
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Number of properties with senior pari passu interest 5cli_RealEstateSalePriceNumberOfPropertiesWithSeniorInterest
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Keystone-TriState [Member] | Principal Balance Due July 1, 2017 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 41,240,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueJulyTwoThousandSeventeenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Property Debt, Maturity Date July 1, 2017        
Property Debt, Interest Rate 4.95%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueJulyTwoThousandSeventeenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Keystone-TriState [Member] | Principal Balance Due September 9, 2017 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 71,172,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueSeptemberNinthTwoThousandSeventeenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Property Debt, Maturity Date September 9, 2017        
Bears interest at fixed rate range, minimum 5.65%us-gaap_DebtInstrumentInterestRateEffectivePercentageRateRangeMinimum
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueSeptemberNinthTwoThousandSeventeenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Bears interest at fixed rate range, maximum 6.75%us-gaap_DebtInstrumentInterestRateEffectivePercentageRateRangeMaximum
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueSeptemberNinthTwoThousandSeventeenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Keystone-TriState [Member] | Principal Balance Due July 6, 2024 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 14,250,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueJulyTwoThousandTwentyFourMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Property Debt, Maturity Date July 6, 2024        
Property Debt, Interest Rate 4.88%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueJulyTwoThousandTwentyFourMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Keystone-TriState [Member] | Principal Balance Due July 6, 2044 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 63,400,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueJulySixthTwoThousandFortyFourMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Property Debt, Maturity Date July 6, 2044        
Property Debt, Interest Rate 4.93%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueJulySixthTwoThousandFortyFourMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Keystone-TriState [Member] | Principal Balance Due August 6, 2044 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 15,050,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustSixthTwoThousandFortyFourMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Property Debt, Maturity Date August 6, 2044        
Property Debt, Interest Rate 4.71%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cli_PrincipalBalanceDueAugustSixthTwoThousandFortyFourMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Keystone-TriState [Member] | Parent Company [Member]          
Schedule of Equity Method Investments [Line Items]          
Internal rate of return 10.00%cli_InternalRateOfReturn
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
Keystone-TriState [Member] | Keystone Entities [Member]          
Schedule of Equity Method Investments [Line Items]          
Internal rate of return 15.00%cli_InternalRateOfReturn
/ dei_LegalEntityAxis
= cli_KeystoneEntitiesMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KeystoneTristateMember
       
KPG-MCG Curtis JV, LLC [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 885,000us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgMcgCurtisJvLLCMember
[25]        
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgMcgCurtisJvLLCMember
[2],[25]        
Carrying Amount 58,341,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgMcgCurtisJvLLCMember
[25]       59,911,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_KpgMcgCurtisJvLLCMember
[25]
Property Debt, Maturity Date (z) [25]        
Property Debt, Interest Rate    [25],[26]        
Plaza VIII & IX Associates, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 1,225,000us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PlazaEightAndNineAssociatesLLCMember
       
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PlazaEightAndNineAssociatesLLCMember
[2]        
Carrying Amount 3,796,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PlazaEightAndNineAssociatesLLCMember
      4,022,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_PlazaEightAndNineAssociatesLLCMember
Property Debt, Maturity Date -        
Roseland/North Retail, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 30,745us-gaap_AreaOfRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandNorthRetailLLCMember
[1]        
Company's Effective Ownership % 20.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandNorthRetailLLCMember
[1],[2]        
Carrying Amount 1,810,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandNorthRetailLLCMember
[1]       1,828,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandNorthRetailLLCMember
[1]
Property Debt, Maturity Date - [1]        
South Pier At Harborside [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 350us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
       
Company's Effective Ownership % 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
[2]        
Carrying Amount    [27]        
Property Debt, Balance 65,308,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
[28]        
Property Debt, Maturity Date (ab)        
Property Debt, Interest Rate           
Accounts payable, accrued expenses and other liabilities 1,937,000us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
      1,854,000us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentAndNoncurrent
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
Letter of credit 4,100,000cli_LineOfCreditFacilityInSupportOfDebtInstrumentFairValueOfAmountOutstanding
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
       
South Pier At Harborside [Member] | 6.15% Mortgage Loan Due November 2016 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 61,184,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_SixPointOneFivePercentMortgageLoanDueNovemberTwoThousandSixteenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
       
Property Debt, Maturity Date November 2016        
Property Debt, Interest Rate 6.15%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= cli_SixPointOneFivePercentMortgageLoanDueNovemberTwoThousandSixteenMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
       
South Pier At Harborside [Member] | Variable Rate Loan Due August 1, 2020 [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 4,100,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_DebtInstrumentAxis
= cli_VariableRateLoanDueAugustFirstTwoThousandTwentyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
       
Property Debt, Maturity Date August 1, 2020        
Bears interest at fixed rate range, minimum 6.09%us-gaap_DebtInstrumentInterestRateEffectivePercentageRateRangeMinimum
/ us-gaap_DebtInstrumentAxis
= cli_VariableRateLoanDueAugustFirstTwoThousandTwentyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
       
Bears interest at fixed rate range, maximum 6.62%us-gaap_DebtInstrumentInterestRateEffectivePercentageRateRangeMaximum
/ us-gaap_DebtInstrumentAxis
= cli_VariableRateLoanDueAugustFirstTwoThousandTwentyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
       
Letter of credit 4,100,000cli_LineOfCreditFacilityInSupportOfDebtInstrumentFairValueOfAmountOutstanding
/ us-gaap_DebtInstrumentAxis
= cli_VariableRateLoanDueAugustFirstTwoThousandTwentyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_SouthPierAtHarborsideMember
       
Stamford SM LLC [Member]          
Schedule of Equity Method Investments [Line Items]          
Company's Effective Ownership % 80.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_StamfordSmLlcMember
[2],[29]        
Property Debt, Maturity Date - [29]        
Stamford SM LLC [Member] | Mezz Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Collection of notes receivable 47,200,000us-gaap_ProceedsFromCollectionOfLoansReceivable
/ us-gaap_LongtermDebtTypeAxis
= cli_MezzLoanMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_StamfordSmLlcMember
       
Stamford SM LLC [Member] | Parent Company [Member] | Mezz Loan [Member]          
Schedule of Equity Method Investments [Line Items]          
Collection of notes receivable 37,800,000us-gaap_ProceedsFromCollectionOfLoansReceivable
/ dei_LegalEntityAxis
= us-gaap_ParentCompanyMember
/ us-gaap_LongtermDebtTypeAxis
= cli_MezzLoanMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_StamfordSmLlcMember
       
Other [Member]          
Schedule of Equity Method Investments [Line Items]          
Carrying Amount 404,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OtherJointVenturesMember
[30]       907,000us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OtherJointVenturesMember
[30]
Property Debt, Maturity Date - [30]        
The Shops At 40 Park Property [Member] | Rosewood Morristown, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Square Feet 50,973us-gaap_AreaOfRealEstateProperty
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_ShopsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Property Debt, Balance 6,500,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_ShopsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Property Debt, Maturity Date August 2018        
Property Debt, Interest Rate 3.63%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_ShopsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Residual ownership interest 12.50%cli_ResidualOwnershipInterest
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_ShopsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Additional borrowing capacity 1,000,000cli_MortgageLoanOnRealEstateAdditionalBorrowingCapacity
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_ShopsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Lofts At 40 Park Property [Member] | Rosewood Morristown, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 59us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_LoftsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Property Debt, Balance 1,117,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_LoftsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Property Debt, Maturity Date September 2015        
Property Debt, Interest Rate, Spread Over LIBOR 2.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_LoftsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Indirect ownership interest 25.00%cli_IndirectOwnershipPercentage
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_LoftsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Number of stories 5cli_NumberOfStoriesInRealEstateProperty
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_LoftsAtFortyParkPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Metropolitan Property [Member] | Rosewood Morristown, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 38,600,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_MetropolitanPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Property Debt, Maturity Date September 2020        
Property Debt, Interest Rate 3.25%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_MetropolitanPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RosewoodMorristownLLCMember
       
Quarrystone I Property [Member] | Overlook Ridge JV, L.L.C. [Member] | Senior Loans [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 52,420,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorLoansMember
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_QuarrystoneIPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJVLLCMember
       
Property Debt, Maturity Date March 2016        
Property Debt, Interest Rate, Spread Over LIBOR 2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorLoansMember
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_QuarrystoneIPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJVLLCMember
       
Quarrystone I Property [Member] | Overlook Ridge JV, L.L.C. [Member] | Junior Loans [Member]          
Schedule of Equity Method Investments [Line Items]          
Property Debt, Balance 17,000,000us-gaap_MortgageLoansOnRealEstateCarryingAmountOfMortgages
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorLoansMember
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_QuarrystoneIPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJVLLCMember
       
Property Debt, Maturity Date March 2016        
Property Debt, Interest Rate, Spread Over LIBOR 0.90%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorLoansMember
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_QuarrystoneIPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJVLLCMember
       
Port Imperial North Land [Member] | Roseland/Port Imperial Partners, L.P. [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 836us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_PortImperialNorthLandMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandPortImperialPartnersLPMember
       
Residual ownership interest 20.00%cli_ResidualOwnershipInterest
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_PortImperialNorthLandMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_RoselandPortImperialPartnersLPMember
       
Letter of Credit [Member] | Quarrystone I Property [Member] | Overlook Ridge JV, L.L.C. [Member]          
Schedule of Equity Method Investments [Line Items]          
Maximum borrowing capacity $ 17,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis
= cli_QuarrystoneIPropertyMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= cli_OverlookRidgeJVLLCMember
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_LetterOfCreditMember
       
Multi-Family Properties [Member]          
Schedule of Equity Method Investments [Line Items]          
Number of Apartment Units 5,861us-gaap_NumberOfUnitsInRealEstateProperty
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= cli_MultipleFamilyPropertiesMember
       
[1] The Company's ownership interests in this venture are subordinate to its partner's preferred capital balance and the Company is not expected to meaningfully participate in the venture's cash flows in the near term.
[2] Company's effective ownership % represents the Company's entitlement to residual distributions after payments of priority returns, where applicable.
[3] Through the joint venture, the Company also owns a 12.5 percent interest in a 50,973 square feet retail building ("Shops at 40 Park") and a 25 percent interest in a to-be-built 59-unit, five story multi-family rental development property ("Lofts at 40 Park").
[4] The Company's ownership interests in this venture are subordinate to its partner's preferred capital balance and the payment of the outstanding balance remaining on a note ($975 as of March 31, 2015), and is not expected to meaningfully participate in the venture's cash flows in the near term.
[5] Property debt balance consists of: (i) a loan, collateralized by the Metropolitan at 40 Park, with a balance of $38,600 at March 31, 2015, bears interest at 3.25 percent, matures in September 2020 and is interest only through September 2015; (ii) a loan, collateralized by the Shops at 40 Park, with a balance of $6,500 at March 31, 2015, bears interest at 3.63 percent, matures in August 2018 and is interest-only through July 2015; and (iii) a loan, collateralized by the Lofts at 40 Park, with a balance of $1,117, bears interest at LIBOR plus 250 basis points and matures in September 2015. The Shops at 40 Park mortgage loan also provides for additional borrowing proceeds of $1 million based on certain preferred thresholds being achieved.
[6] Property debt balance consists of: (i) the senior loan, collateralized by the Quarrystone property, with a balance of $52,420 at March 31, 2015, bears interest at LIBOR plus 200 basis, matures in March 2016 and (ii) the junior loan, with a balance of $17,000, bears interest at LIBOR plus 90 basis points, matures in March 2016 and is collateralized by a $17,000 letter of credit provided by an affiliate of the partner.
[7] The construction loan has a maximum borrowing amount of $55,500 and provides, subject to certain conditions, two one-year extension options with a fee of 25 basis points each. The joint venture has a swap agreement that fixes the all-in rate to 3.0875 percent per annum on an initial notional amount of $1,840, increasing to $52,000, for the period from September 3, 2013 to November 2, 2015.
[8] The permanent loan has a maximum borrowing amount of $80,249.
[9] The construction loan with a maximum borrowing amount of $91,000 converted to a permanent loan on February 27, 2015.
[10] The Company also owns a 50 percent interest in a vacant land to accommodate the development of approximately 295 additional units of which 252 are currently approved.
[11] The construction loan has a maximum borrowing amount of $42,500 and provides, subject to certain conditions, two two-year extension options with a fee of 12.5 basis points for the first two-year extension and 25 basis points for the second two-year extension.
[12] The construction loan has a maximum borrowing amount of $73,350 and provides, subject to certain conditions, one-year extension option followed by a six-month extension option with a fee of 25 basis points each. The joint venture has a swap agreement that fixes the all-in rate to 2.79 percent per annum on an initial notional amount of $1,620, increasing to $69,500 for the period from July 1, 2013 to January 1, 2016.
[13] The Company also owns a 20 percent residual interest in undeveloped land parcels: parcels 6, I, and J ("Port Imperial North Land") that can accommodate the development of 836 apartment units.
[14] The construction loan has a maximum borrowing amount of $77,400 and provides, subject to certain conditions, two one-year extension options with a fee of 25 basis points for each year.
[15] The construction loan with a maximum borrowing amount of $57,000 converted to a permanent loan on February 27, 2015.
[16] The construction loan has a maximum borrowing amount of $23,400 and provides, subject to certain conditions, two one-year extension options with a fee of 20 basis points for each year.
[17] The construction/permanent loan has a maximum borrowing amount of $100,700 with amortization starting in August 2017.
[18] The construction/permanent loan has a maximum borrowing amount of $192,000.
[19] The joint venture has a swap agreement that fixes the all-in rate to 3.99375 percent per annum on an initial notional amount of $13,650 and then adjusting in accordance with an amortization schedule, which is effective from October 17, 2011 through loan maturity.
[20] The mortgage loan has two one-year extension options, subject to certain conditions, and includes a $25 million construction escrow with a balance of $13.6 million to be drawn at March 31, 2015.
[21] The Company’s equity interests in the joint ventures will be subordinated to Keystone Entities receiving a 15 percent internal rate of return (“IRR”) after which the Company will receive a 10 percent IRR on its subordinate equity and then all profit will be split equally.
[22] Principal balance of $127,600 bears interest at 5.114 percent and matures in August 27, 2023; principal balance of $67,921 bears interest at rates ranging from LIBOR+5.0 percent to LIBOR+5.75 percent and matures in August 27, 2016; principal balance of $10,425 bears interest at LIBOR+6.0 percent matures in August 27, 2015.
[23] Includes the Company’s pari-passu interests of $4.8 million in five properties and Company’s subordinated equity interests to Keystone Entities receiving a 15 percent internal rate of return (“IRR”) after which the Company will receive a 10 percent IRR on its subordinate equity and then all profit will be split equally.
[24] Principal balance of $41,240 bears interest at 4.95 percent and matures on July 1, 2017; principal balance of $71,172 bears interest at rates ranging from 5.65 percent to 6.75 percent and matures on September 9, 2017; principal balance of $14,250 bears interest at 4.88 percent and matures on July 6, 2024; principal balance of $63,400 bears interest at 4.93 percent and matures on July 6, 2044; principal balance of $15,050 bears interest at 4.71 percent and matures on August 6, 2044.
[25] Includes undivided interests in the same manner as investments in noncontrolling partnership, pursuant to ASC 970-323-25-12.
[26] See Note 9: Mortgages, Loans Payable and Other Obligations for debt secured by interests in these assets.
[27] The negative carrying amount for this venture of $1,937 and $1,854 as of March 31, 2015 and December 31, 2014, respectively, were included in accounts payable, accrued expenses and other liabilities.
[28] Balance includes: (i) mortgage loan, collateralized by the hotel property, with a balance of $61,184, bears interest at 6.15 percent and matures in November 2016, and (ii) loan with a balance of $4.1 million, bears interest at fixed rates ranging from 6.09 percent to 6.62 percent and matures in August 1, 2020. The Company posted a $4.1 million letter of credit in support of this loan, half of which is indemnified by the partner.
[29] The joint venture collected net proceeds of $47.2 million at maturity, of which the Company received its share of $37.8 million on August 6, 2014.
[30] The Company owns other interests in various unconsolidated joint ventures, including interests in assets previously owned and interest in ventures whose businesses are related to its core operations. These ventures are not expected to significantly impact the Company's operations in the near term.