Mortgage Loans on Real Estate, Senior Financing

No definition available.

Name:	slg_MortgageLoansonRealEstateSeniorFinancing
Namespace Prefix:	slg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Rate of interest on investment.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

Name:	us-gaap_InvestmentInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Nominal or face amount on the investment owned.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 20
-Section 10
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6438156&loc=d3e57880-113973

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 10
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6431724&loc=d3e32938-113948

+ Details

Name:	us-gaap_InvestmentOwnedUnderlyingFaceAmountAtMarketValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The percentage points (also referred to as 'margin') added to the reference rate as stated in the loan agreement and used to compute the variable rate on the loans receivable. For instance, the loan agreement might state that the interest rate the borrower is obligated to pay is the London Interbank Offered Rate (LIBOR) plus three quarters (3/4) of a percentage point adjusted quarterly (each three months). LIBOR in this example is the index or reference rate, 3/4 percentage point is the margin, the reference rate plus the margin is the fully indexed rate. This element would be used to report separately the margin.

+ References

No definition available.

+ Details

Name:	us-gaap_LoansReceivableBasisSpreadOnVariableRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(c))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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- Details

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=slg_JuniorMortgageMember
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Data Type:	na
Balance Type:
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- Details

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=slg_MezzanineDebtMember
Namespace Prefix:
Data Type:	na
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- Details

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=slg_PreferredEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=slg_DebtInvestmentWithFloatingInterestRateMember
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- Details

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Debt and Preferred Equity Investments - Debt and Preferred Equity Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Carrying value	$ 1,580,306	$ 2,099,393
Senior Financing	5,560,939
Senior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	342,523
Senior Financing	0
Junior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	31,960
Senior Financing	100,000
Mezzanine Debt
Debt Instrument [Line Items]
Carrying value	966,587
Senior Financing	5,188,939
Preferred Equity
Debt Instrument [Line Items]
Carrying value	239,236
Senior Financing	272,000
Total floating rate
Debt Instrument [Line Items]
Carrying value	618,885
Face Value	622,246
Total floating rate \| Senior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	341,513
Face Value	342,861
Total floating rate \| Junior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	31,960
Face Value	32,000
Total floating rate \| Mezzanine Debt
Debt Instrument [Line Items]
Carrying value	245,412
Face Value	247,385
Total floating rate \| Preferred Equity
Debt Instrument [Line Items]
Carrying value	0
Face Value	$ 0
Total floating rate \| LIBOR \| Minimum \| Senior Mortgage Debt
Debt Instrument [Line Items]
Basis spread on variable rate	2.75%
Total floating rate \| LIBOR \| Minimum \| Junior Mortgage Debt
Debt Instrument [Line Items]
Basis spread on variable rate	6.00%
Total floating rate \| LIBOR \| Minimum \| Mezzanine Debt
Debt Instrument [Line Items]
Basis spread on variable rate	4.60%
Total floating rate \| LIBOR \| Maximum \| Senior Mortgage Debt
Debt Instrument [Line Items]
Basis spread on variable rate	5.50%
Total floating rate \| LIBOR \| Maximum \| Junior Mortgage Debt
Debt Instrument [Line Items]
Basis spread on variable rate	7.25%
Total floating rate \| LIBOR \| Maximum \| Mezzanine Debt
Debt Instrument [Line Items]
Basis spread on variable rate	12.36%
Total fixed rate
Debt Instrument [Line Items]
Carrying value	$ 961,421
Face Value	974,371
Total fixed rate \| Senior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	1,010
Face Value	1,250
Total fixed rate \| Junior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	0
Face Value	0
Total fixed rate \| Mezzanine Debt
Debt Instrument [Line Items]
Carrying value	721,175
Face Value	728,138
Total fixed rate \| Preferred Equity
Debt Instrument [Line Items]
Carrying value	239,236
Face Value	$ 244,983
Total fixed rate \| Minimum \| Mezzanine Debt
Debt Instrument [Line Items]
Interest rate	2.90%
Total fixed rate \| Minimum \| Preferred Equity
Debt Instrument [Line Items]
Interest rate	7.00%
Total fixed rate \| Maximum \| Mezzanine Debt
Debt Instrument [Line Items]
Interest rate	10.00%
Total fixed rate \| Maximum \| Preferred Equity
Debt Instrument [Line Items]
Interest rate	11.00%