Debt Instrument, Basis Spread On Variable Rate, Increase (Decrease)

No definition available.

Name:	slg_DebtInstrumentBasisSpreadOnVariableRateIncreaseDecrease
Namespace Prefix:	slg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line Of Credit Facility, Number Of Extensions

+ References

No definition available.

+ Details

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Namespace Prefix:	slg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

The expanded maximum borrowing capacity available, which is subject to agreement with the entity.

+ References

No definition available.

+ Details

Name:	slg_LineOfCreditFacilityOptionalExpansionMaximumBorrowingCapacity
Namespace Prefix:	slg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Term Of Extension

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
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- 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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Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: 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(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: 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(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

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- Details

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- Details

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- Details

Name:	us-gaap_DebtInstrumentAxis=us-gaap_RevolvingCreditFacilityMember
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- Details

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- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
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- Details

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- Details

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- Details

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Corporate Indebtedness - Additional Information (Details)	1 Months Ended	9 Months Ended
	May 31, 2019	Sep. 30, 2019 USD ($) extension	Dec. 31, 2018 USD ($)
Corporate Indebtedness
Outstanding under line of credit facility		$ 328,549,000	$ 492,196,000
Term Loan B, Maturity November 21, 2024 \| Term loan
Corporate Indebtedness
Credit facility, maximum borrowing capacity		$ 200,000,000.0
Term Loan B, Maturity November 21, 2024 \| LIBOR
Corporate Indebtedness
Decrease in the interest rate spread	0.65%
Term Loan B, Maturity November 21, 2024 \| LIBOR \| Term loan
Corporate Indebtedness
Interest rate added to base rate (as a percent)		1.65%
Term Loan B, Maturity November 21, 2024 \| LIBOR \| Minimum
Corporate Indebtedness
Interest rate added to base rate (as a percent)	0.85%
Term Loan B, Maturity November 21, 2024 \| LIBOR \| Minimum \| Term loan
Corporate Indebtedness
Interest rate added to base rate (as a percent)		1.50%
Term Loan B, Maturity November 21, 2024 \| LIBOR \| Maximum
Corporate Indebtedness
Interest rate added to base rate (as a percent)	1.65%
Term Loan B, Maturity November 21, 2024 \| LIBOR \| Maximum \| Term loan
Corporate Indebtedness
Interest rate added to base rate (as a percent)		2.45%
Revolving credit facility
Corporate Indebtedness
Facility fee (as a percent)		0.20%
Outstanding under line of credit facility		$ 335,000,000.0
Revolving credit facility \| Minimum
Corporate Indebtedness
Facility fee (as a percent)		0.125%
Revolving credit facility \| Maximum
Corporate Indebtedness
Facility fee (as a percent)		0.30%
Term loan
Corporate Indebtedness
Credit facility, maximum borrowing capacity		$ 1,500,000,000
Term loan \| Line of Credit
Corporate Indebtedness
Long-term debt, carrying value		1,500,000,000	1,500,000,000
2012 Credit Facility
Corporate Indebtedness
Letters of credit		11,800,000
Ability to borrow under line of credit facility		1,200,000,000
2012 Credit Facility \| Line of Credit
Corporate Indebtedness
Long-term debt, carrying value		328,500,000	$ 492,200,000
Term Loan A, Maturity March 31, 2023 \| Term loan
Corporate Indebtedness
Credit facility, maximum borrowing capacity		$ 1,300,000,000
Term Loan A, Maturity March 31, 2023 \| LIBOR \| Term loan
Corporate Indebtedness
Interest rate added to base rate (as a percent)		1.10%
Term Loan A, Maturity March 31, 2023 \| LIBOR \| Minimum \| Term loan
Corporate Indebtedness
Interest rate added to base rate (as a percent)		0.90%
Term Loan A, Maturity March 31, 2023 \| LIBOR \| Maximum \| Term loan
Corporate Indebtedness
Interest rate added to base rate (as a percent)		1.75%
Revolving credit facility \| Revolving Credit Facility, Maturity March 31, 2022 \| Line of Credit
Corporate Indebtedness
Credit facility, maximum borrowing capacity		$ 1,500,000,000
Number of extensions \| extension		2
Term of extension		6 months
Maximum borrowing capacity, optional expansion		$ 4,500,000,000
Revolving credit facility \| Revolving Credit Facility, Maturity March 31, 2022 \| LIBOR \| Line of Credit
Corporate Indebtedness
Interest rate added to base rate (as a percent)		1.00%
Revolving credit facility \| Revolving Credit Facility, Maturity March 31, 2022 \| LIBOR \| Minimum \| Line of Credit
Corporate Indebtedness
Interest rate added to base rate (as a percent)		0.825%
Revolving credit facility \| Revolving Credit Facility, Maturity March 31, 2022 \| LIBOR \| Maximum \| Line of Credit
Corporate Indebtedness
Interest rate added to base rate (as a percent)		1.55%