Represents the number of portfolio segments of financing receivables.

No definition available.

Name:	slg_FinancingReceivablePortfolioSegmentsNumber
Namespace Prefix:	slg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the amount of financing receivables recorded in other assets on the entity's balance sheet.

+ References

No definition available.

+ Details

Name:	slg_FinancingReceivableRecordedInOtherAssets
Namespace Prefix:	slg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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://www.xbrl.org/2003/role/presentationRef
-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=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Namespace Prefix:	us-gaap_
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

Name:	us-gaap_MortgageLoansOnRealEstateLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash outflow for the purchase of loan receivable arising from the financing of goods and services.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3213-108585

+ Details

Name:	us-gaap_PaymentsToAcquireLoansReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_MortgageLoansOnRealEstateLoanTypeAxis=slg_MortgageandJrMortgageLoanwithanInitialMaturityDateofApril2017Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MortgageLoansOnRealEstateLoanTypeAxis=slg_JuniorMortgageParticipationAcquiredinSeptember2014Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt and Preferred Equity Investments - Narrative (Details)	6 Months Ended		37 Months Ended
	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Preferred equity investment
Carrying Value	$ 2,168,515,000			$ 2,114,041,000
Number of portfolio segments of financial receivables (segment) \| segment	1	1
Additional amount of financing receivables included in other assets	$ 89,600,000			65,500,000
Mezzanine/Jr. Mortgage Loan with an Initial Maturity Date of April 2017
Preferred equity investment
Carrying Value	250,500,000			0	$ 0
Junior Mortgage Participation Acquired in September 2014
Preferred equity investment
Payments to acquire loans			$ 0
Carrying Value	$ 0			$ 0	$ 0