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Debt and Preferred Equity Investments (Details 4) (USD $)
 12 Months Ended
Dec. 31, 2014
segment
Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]    
Number of portfolio segments of financial receivables (segment) 1slg_FinancingReceivablePortfolioSegmentsNumber  
Additional amount of financing receivables included in other assets $ 133,500,000slg_FinancingReceivableRecordedInOtherAssets $ 172,800,000slg_FinancingReceivableRecordedInOtherAssets
Unpaid Principal Balance    
With no related allowance recorded   0us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance
With an allowance recorded   10,750,000us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance [1]
Unpaid Principal Balance   10,750,000us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Recorded Investment    
With no related allowance recorded   0us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
With an allowance recorded   10,750,000us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment [1]
Recorded Investment   10,750,000us-gaap_ImpairedFinancingReceivableRecordedInvestment
Allowance Allocated   1,000,000us-gaap_ImpairedFinancingReceivableRelatedAllowance [1]
Average recorded investment in impaired loans   10,900,000us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Investment income recognized   $ 7,100,000slg_ImpairedFinancingReceivableInvestmentIncome
[1] This loan was repaid during the year ended December 31, 2014.