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Investments in Unconsolidated Joint Ventures Investment in Unconsolidated Joint Ventures- Additional Information (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
717 Fifth Avenue      
Investment in Unconsolidated Joint Ventures      
Other income recognized on distribution     $ 67,900,000slg_CashIncomeRecognizedOnDistribution
/ slg_RealEstateByLocationAxis
= slg_FifthAvenue717Member
Joint venture      
Investment in Unconsolidated Joint Ventures      
Management fees, base revenue 16,900,000us-gaap_ManagementFeesBaseRevenue
/ dei_LegalEntityAxis
= us-gaap_CorporateJointVentureMember		 4,700,000us-gaap_ManagementFeesBaseRevenue
/ dei_LegalEntityAxis
= us-gaap_CorporateJointVentureMember		 7,900,000us-gaap_ManagementFeesBaseRevenue
/ dei_LegalEntityAxis
= us-gaap_CorporateJointVentureMember
The Meadows | Joint venture      
Investment in Unconsolidated Joint Ventures      
Loss on early extinguishment of debt     $ 10,800,000slg_IncomeLossFromEquityMethodInvestmentsGainsLossesOnExtinguishmentOfDebt
/ dei_LegalEntityAxis
= us-gaap_CorporateJointVentureMember
/ us-gaap_StatementGeographicalAxis
= slg_TheMeadowsMember