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
Balance Type:	debit
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:
Period Type:

- 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
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- Details

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Debt and Preferred Equity Investments - Debt and Preferred Equity Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Carrying Value	$ 1,783,336	$ 1,580,306
Senior Financing	6,129,560
Total floating rate
Debt Instrument [Line Items]
Carrying Value	823,604
Face Value	839,400
Total fixed rate
Debt Instrument [Line Items]
Carrying Value	959,732
Face Value	998,123
Senior Mortgage Debt
Debt Instrument [Line Items]
Carrying Value	472,749
Senior Financing	0
Senior Mortgage Debt \| Total floating rate
Debt Instrument [Line Items]
Carrying Value	372,547
Face Value	374,002
Senior Mortgage Debt \| Total fixed rate
Debt Instrument [Line Items]
Carrying Value	100,202
Face Value	101,250
Junior Mortgage Debt
Debt Instrument [Line Items]
Carrying Value	83,004
Senior Financing	438,138
Junior Mortgage Debt \| Total floating rate
Debt Instrument [Line Items]
Carrying Value	83,004
Face Value	90,300
Junior Mortgage Debt \| Total fixed rate
Debt Instrument [Line Items]
Carrying Value	0
Face Value	0
Mezzanine Debt
Debt Instrument [Line Items]
Carrying Value	859,891
Senior Financing	5,169,422
Mezzanine Debt \| Total floating rate
Debt Instrument [Line Items]
Carrying Value	368,053
Face Value	375,098
Mezzanine Debt \| Total fixed rate
Debt Instrument [Line Items]
Carrying Value	491,838
Face Value	509,920
Preferred Equity
Debt Instrument [Line Items]
Carrying Value	367,692
Senior Financing	522,000
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	367,692
Face Value	$ 386,953
Minimum \| Senior Mortgage Debt \| Total fixed rate
Debt Instrument [Line Items]
Interest rate	3.00%
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 \| Senior Mortgage Debt \| Total fixed rate
Debt Instrument [Line Items]
Interest rate	6.00%
Maximum \| Mezzanine Debt \| Total fixed rate
Debt Instrument [Line Items]
Interest rate	9.50%
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	5.50%
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	15.29%