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

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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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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- Details

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- Details

Name:	us-gaap_InvestmentTypeAxis=slg_FixedRateMember
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- Details

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

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=slg_JuniorMortgageMember
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- Details

Name:	srt_MortgageLoansOnRealEstateLoanTypeAxis=slg_MezzanineDebtMember
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- Details

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Debt and Preferred Equity Investments - Debt and Preferred Equity Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Carrying Value	$ 1,153,363	$ 1,580,306
Senior Financing
Total floating rate
Debt Instrument [Line Items]
Carrying Value	426,704
Face Value	443,458
Total fixed rate
Debt Instrument [Line Items]
Carrying Value	726,659
Face Value	740,946
Senior Mortgage Debt
Debt Instrument [Line Items]
Carrying Value	63,932
Senior Financing	0
Senior Mortgage Debt \| Total floating rate
Debt Instrument [Line Items]
Carrying Value	62,683
Face Value	63,425
Senior Mortgage Debt \| Total fixed rate
Debt Instrument [Line Items]
Carrying Value	1,249
Face Value	$ 1,250
Interest rate	3.00%
Junior Mortgage Debt
Debt Instrument [Line Items]
Carrying Value	$ 118,100
Senior Financing	542,241
Junior Mortgage Debt \| Total floating rate
Debt Instrument [Line Items]
Carrying Value	85,295
Face Value	96,697
Junior Mortgage Debt \| Total fixed rate
Debt Instrument [Line Items]
Carrying Value	32,805
Face Value	$ 33,000
Interest rate	6.00%
Mezzanine Debt
Debt Instrument [Line Items]
Carrying Value	$ 714,766
Senior Financing	4,447,803
Mezzanine Debt \| Total floating rate
Debt Instrument [Line Items]
Carrying Value	278,726
Face Value	283,336
Mezzanine Debt \| Total fixed rate
Debt Instrument [Line Items]
Carrying Value	436,040
Face Value	447,265
Preferred Equity
Debt Instrument [Line Items]
Carrying Value	256,565
Senior Financing	1,962,750
Preferred Equity \| Total floating rate
Debt Instrument [Line Items]
Carrying Value	0
Face Value	0
Preferred Equity \| Total fixed rate
Debt Instrument [Line Items]
Carrying Value	256,565
Face Value	$ 259,431
Minimum \| Mezzanine Debt \| Total fixed rate
Debt Instrument [Line Items]
Interest rate	2.90%
Minimum \| Preferred Equity \| Total fixed rate
Debt Instrument [Line Items]
Interest rate	6.50%
Maximum \| Mezzanine Debt \| Total fixed rate
Debt Instrument [Line Items]
Interest rate	9.30%
Maximum \| Preferred Equity \| Total fixed rate
Debt Instrument [Line Items]
Interest rate	11.00%
LIBOR \| Minimum \| Senior Mortgage Debt \| Total floating rate
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
LIBOR \| Minimum \| Junior Mortgage Debt \| Total floating rate
Debt Instrument [Line Items]
Basis spread on variable rate	6.00%
LIBOR \| Minimum \| Mezzanine Debt \| Total floating rate
Debt Instrument [Line Items]
Basis spread on variable rate	4.95%
LIBOR \| Maximum \| Senior Mortgage Debt \| Total floating rate
Debt Instrument [Line Items]
Basis spread on variable rate	3.50%
LIBOR \| Maximum \| Junior Mortgage Debt \| Total floating rate
Debt Instrument [Line Items]
Basis spread on variable rate	7.25%
LIBOR \| Maximum \| Mezzanine Debt \| Total floating rate
Debt Instrument [Line Items]
Basis spread on variable rate	14.62%