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Significant Accounting Policies (Details 3) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2012
Dec. 31, 2013
Investment in Marketable Securities      
Marketable securities $ 39,429,000us-gaap_AvailableForSaleSecurities   $ 32,049,000us-gaap_AvailableForSaleSecurities
Net proceeds on sale of marketable securities 4,400,000slg_ProceedsfromSaleandMaturityofMarketableSecuritiesNetofTransactionCosts 6,800,000slg_ProceedsfromSaleandMaturityofMarketableSecuritiesNetofTransactionCosts  
Realized gain (loss) on investment in marketable securities 3,900,000us-gaap_MarketableSecuritiesRealizedGainLoss 4,900,000us-gaap_MarketableSecuritiesRealizedGainLoss  
Commercial mortgage-backed securities      
Investment in Marketable Securities      
Cost basis 32,400,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
  23,000,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Rake bonds      
Investment in Marketable Securities      
Cost basis     3,600,000us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= slg_RakeBondsMember
Level 1      
Investment in Marketable Securities      
Marketable securities 4,332,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
  4,307,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
Level 1 | Equity marketable securities      
Investment in Marketable Securities      
Marketable securities 4,332,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
  4,307,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel1Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Level 2      
Investment in Marketable Securities      
Marketable securities 35,097,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
  24,419,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
Level 2 | Commercial mortgage-backed securities      
Investment in Marketable Securities      
Marketable securities 35,097,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
  24,419,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
Level 3      
Investment in Marketable Securities      
Marketable securities 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
  3,323,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
Level 3 | Rake bonds      
Investment in Marketable Securities      
Marketable securities 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= slg_RakeBondsMember
  3,323,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= slg_RakeBondsMember
Fair Value      
Investment in Marketable Securities      
Marketable securities $ 39,429,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember
  $ 32,049,000us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember