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Fair Value Measurements (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Fair Value of Financial Instruments    
Marketable securities $ 39,429,000us-gaap_AvailableForSaleSecurities $ 32,049,000us-gaap_AvailableForSaleSecurities
Interest rate swap agreements (included in other assets) 2,174,000us-gaap_DerivativeFairValueOfDerivativeAsset  
Interest rate swap agreements (included in accrued interest payable and other liabilities) 14,728,000us-gaap_DerivativeFairValueOfDerivativeLiability 1,329,000us-gaap_DerivativeFairValueOfDerivativeLiability
Carrying Value, Net of Discounts and Deferred Origination Fees 1,408,804,000us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet 1,304,839,000us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Carrying Value    
Fair Value of Financial Instruments    
Fixed rate debt 6,140,786,000us-gaap_LongtermDebtPercentageBearingFixedInterestAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 5,599,960,000us-gaap_LongtermDebtPercentageBearingFixedInterestAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Variable rate debt 2,291,943,000us-gaap_LongtermDebtPercentageBearingVariableInterestAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 1,319,948,000us-gaap_LongtermDebtPercentageBearingVariableInterestAmount
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Total 8,432,729,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 6,919,908,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember
Fair Value    
Fair Value of Financial Instruments    
Total 8,881,188,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 7,214,402,000us-gaap_LongTermDebtFairValue
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Estimated fair value of debt and preferred equity investments, low end of range 1,500,000,000slg_MortgageLoansOnRealEstateFairValueLowEndOfRange
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 1,300,000,000slg_MortgageLoansOnRealEstateFairValueLowEndOfRange
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Estimated fair value of debt and preferred equity investments, high end of range 1,800,000,000slg_MortgageLoansOnRealEstateFairValueHighEndOfRange
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 1,400,000,000slg_MortgageLoansOnRealEstateFairValueHighEndOfRange
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Level 1    
Fair Value of Financial Instruments    
Marketable securities 4,332,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 4,307,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Interest rate swap agreements (included in other assets) 0us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		  
Interest rate swap agreements (included in accrued interest payable and other liabilities) 0us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member		 0us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Level 2    
Fair Value of Financial Instruments    
Marketable securities 35,097,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 24,419,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Interest rate swap agreements (included in other assets) 2,174,000us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		  
Interest rate swap agreements (included in accrued interest payable and other liabilities) 14,728,000us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 1,329,000us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Level 3    
Fair Value of Financial Instruments    
Marketable securities 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 3,323,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Interest rate swap agreements (included in other assets) 0us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		  
Interest rate swap agreements (included in accrued interest payable and other liabilities) 0us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member		 0us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Level 3 | Carrying Value    
Fair Value of Financial Instruments    
Carrying Value, Net of Discounts and Deferred Origination Fees 1,408,804,000us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 [1] 1,304,839,000us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_CarryingReportedAmountFairValueDisclosureMember		 [1]
Level 3 | Fair Value    
Fair Value of Financial Instruments    
Fixed rate debt 6,565,236,000us-gaap_LongtermDebtPercentageBearingFixedInterestAmount
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 5,886,980,000us-gaap_LongtermDebtPercentageBearingFixedInterestAmount
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
Variable rate debt $ 2,315,952,000us-gaap_LongtermDebtPercentageBearingVariableInterestAmount
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		 $ 1,327,422,000us-gaap_LongtermDebtPercentageBearingVariableInterestAmount
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
[1] At December 31, 2014, debt and preferred equity investments had an estimated fair value ranging between $1.5 billion and $1.8 billion. At December 31, 2013, debt and preferred equity investments had an estimated fair value ranging between $1.3 billion and $1.4 billion.