Represents the ownership percentage of the joint venture sold to the incoming partner.

No definition available.

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

- Definition

Properties Sold [Abstract]

+ References

No definition available.

+ Details

Name:	slg_PropertiesSoldAbstract
Namespace Prefix:	slg_
Data Type:	xbrli:stringItemType
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

Amount after tax of gain (loss) not previously recognized resulting from the disposal of a discontinued operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1361-107760

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 3B
-URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721525-107759

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5C
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721675-107760

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e957-107759

+ Details

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

- Definition

Amount of consideration received or receivable for the disposal of assets and liabilities, including discontinued operation.

+ References

No definition available.

+ Details

Name:	us-gaap_DisposalGroupIncludingDiscontinuedOperationConsideration
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The aggregate amount of expenditures for salaries, wages, profit sharing and incentive compensation, and other employee benefits, including equity-based compensation, and pension and other postretirement benefit expense.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.4)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_LaborAndRelatedExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=slg_ThirdAvenue885Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=slg_WhitePlainsRoad520Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=slg_GreeneStreet102Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=slg_A680750WashingtonBoulevardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=slg_CourtStreet16Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Properties Held for Sale and Property Dispositions (Details) $ in Millions	1 Months Ended				9 Months Ended
	Sep. 30, 2017 ft²	Jul. 31, 2017 USD ($) ft²	Apr. 30, 2017 USD ($) ft²	Feb. 29, 2016 USD ($) ft²	Sep. 30, 2017 USD ($) ft²	Oct. 31, 2017 USD ($)
Properties Sold [Abstract]
Approximate Square Feet \| ft²	31,800,088				31,800,088
885 Third Avenue
Properties Sold [Abstract]
Approximate Square Feet \| ft²				607,000
Sales Price				$ 453.0
Gain on Sale (Loss)				$ (8.8)
520 White Plains Road
Properties Sold [Abstract]
Approximate Square Feet \| ft²			180,000
Sales Price			$ 21.0
Gain on Sale (Loss)			$ (14.6)
102 Greene Street
Properties Sold [Abstract]
Approximate Square Feet \| ft²			9,200
Sales Price			$ 43.5
Gain on Sale (Loss)			$ 4.9
Employee compensation award					$ 0.9
Joint venture, ownership percentage sold	10.00%		90.00%
680/750 Washington Boulevard
Properties Sold [Abstract]
Approximate Square Feet \| ft²		325,000
Sales Price		$ 97.0
Gain on Sale (Loss)		$ (44.2)
Subsequent Event \| 16 Court
Properties Sold [Abstract]
Sales Price						$ 171.0