Disposal Group Including Discontinued Operation, Ownership Percentage in Disposed Asset

No definition available.

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

- Definition

Fee Interest Acquired, Number Of Land Parcels

+ References

No definition available.

+ Details

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

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
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_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of consideration transferred, consisting of acquisition-date fair value of assets transferred by the acquirer, liabilities incurred by the acquirer, and equity interest issued by the acquirer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6613-128477

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120321790&loc=d3e6927-128479

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 30
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=116859721&loc=d3e6578-128477

+ Details

Name:	us-gaap_BusinessCombinationConsiderationTransferred1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to items of consideration transferred in connection with a business combination for which the initial accounting was incomplete.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 4A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121647850&loc=SL65897772-128472

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=slg_A2HeraldSquareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=slg_A1231ThirdAvenueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=slg_UpperEastSideResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=slg_A113GreeneStreetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BusinessAcquisitionAxis=slg_A712MadisonAvenueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=slg_A2HeraldSquareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Property Acquisitions - 2018 Acquisitions (Details) $ in Millions	1 Months Ended
	Dec. 31, 2018 USD ($)	Nov. 30, 2018 ft²	Oct. 31, 2018 USD ($) ft²	Aug. 31, 2018 USD ($) a parcel	Jul. 31, 2018 USD ($) ft²	May 31, 2018 USD ($) ft²	Dec. 31, 2020 ft²	Dec. 31, 2019 ft²
Business Acquisition [Line Items]
Approximate Square Feet \| ft²							29,500,737
2 Herald Square
Business Acquisition [Line Items]
Approximate Square Feet \| ft²						369,000
Acquisition Price (in millions) \| $						$ 266.0
Purchase price fair value adjustment \| $						$ 8.1
1231 Third Avenue
Business Acquisition [Line Items]
Approximate Square Feet \| ft²					39,000
Acquisition Price (in millions) \| $					$ 55.4
Upper East Side Residential
Business Acquisition [Line Items]
Approximate Square Feet \| a				0.2
Acquisition Price (in millions) \| $				$ 30.2
Number of land parcels, fee interest acquired \| parcel				3
113 Greene Street
Business Acquisition [Line Items]
Approximate Square Feet \| ft²			6,425
Acquisition Price (in millions) \| $			$ 31.0
712 Madison Avenue
Business Acquisition [Line Items]
Approximate Square Feet \| ft²								6,600
Acquisition Price (in millions) \| $	$ 58.0
2 Herald Square
Business Acquisition [Line Items]
Approximate Square Feet \| ft²		369,000
Ownership percentage in disposed asset		49.00%