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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Namespace Prefix:	us-gaap_
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=121645371&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=121645371&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=120400017&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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Period Type:

- 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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Name:	us-gaap_InvestmentTypeAxis=slg_FixedRateMember
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Debt and Preferred Equity Investments - Debt and Preferred Equity Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Carrying value	$ 1,076,542	$ 1,580,306
Senior Financing	6,549,037
Senior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	64,000
Senior Financing	0
Junior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	40,088
Senior Financing	127,000
Mezzanine Debt
Debt Instrument [Line Items]
Carrying value	712,668
Senior Financing	4,459,287
Preferred Equity
Debt Instrument [Line Items]
Carrying value	259,786
Senior Financing	1,962,750
Total floating rate
Debt Instrument [Line Items]
Carrying value	345,877
Face Value	355,544
Total floating rate \| Senior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	62,751
Face Value	63,425
Total floating rate \| Junior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	7,200
Face Value	12,000
Total floating rate \| Mezzanine Debt
Debt Instrument [Line Items]
Carrying value	275,926
Face Value	280,119
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	2.00%
Total floating rate \| LIBOR \| Minimum \| Junior Mortgage Debt
Debt Instrument [Line Items]
Basis spread on variable rate	7.25%
Total floating rate \| LIBOR \| Minimum \| Mezzanine Debt
Debt Instrument [Line Items]
Basis spread on variable rate	4.95%
Total floating rate \| LIBOR \| Maximum \| Senior Mortgage Debt
Debt Instrument [Line Items]
Basis spread on variable rate	3.50%
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	14.07%
Total fixed rate
Debt Instrument [Line Items]
Carrying value	$ 730,665
Face Value	745,442
Total fixed rate \| Senior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	1,249
Face Value	$ 1,250
Interest rate	3.50%
Total fixed rate \| Junior Mortgage Debt
Debt Instrument [Line Items]
Carrying value	$ 32,888
Face Value	$ 33,000
Interest rate	6.00%
Total fixed rate \| Mezzanine Debt
Debt Instrument [Line Items]
Carrying value	$ 436,742
Face Value	448,938
Total fixed rate \| Preferred Equity
Debt Instrument [Line Items]
Carrying value	259,786
Face Value	$ 262,254
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	6.50%
Total fixed rate \| Maximum \| Mezzanine Debt
Debt Instrument [Line Items]
Interest rate	14.30%
Total fixed rate \| Maximum \| Preferred Equity
Debt Instrument [Line Items]
Interest rate	11.00%