Disposal Group, Not Discontinued Operation, Number of Properties

No definition available.

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

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

Name:	slg_FiniteLivedIntangibleLiabilitiesAbstract
Namespace Prefix:	slg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the accumulated amount of amortization of a major finite-lived intangible liability class.

+ References

No definition available.

+ Details

Name:	slg_FiniteLivedIntangibleLiabilitiesAccumulatedAmortization
Namespace Prefix:	slg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the gross carrying amounts before accumulated amortization as of the balance sheet date of all intangible liabilities having statutory or estimated useful lives.

+ References

No definition available.

+ Details

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

- Definition

Represents the sum of gross carrying value of a major finite-lived intangible liability class, less accumulated amortization.

+ References

No definition available.

+ Details

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

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

Name:	slg_FinitelyLivedIntangibleAssetsAbstract
Namespace Prefix:	slg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the increase (decrease) in interest expense during the period resulting from the amortization of assumed above-market rate mortgages.

+ References

No definition available.

+ Details

Name:	slg_InterestExpenseIncreaseDecreaseAssumedAboveMarketRateMortgageAmortization
Namespace Prefix:	slg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Net Intangible Assets Transferred to Assets Held for Sale

+ References

No definition available.

+ Details

Name:	slg_NetIntangibleAssetsTransferredtoAssetsHeldforSale
Namespace Prefix:	slg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Net Intangible Liabilities Transferred to Liabilities Related to Assets Held for Sale

+ References

No definition available.

+ Details

Name:	slg_NetIntangibleLiabilitiesTransferredtoLiabilitiesRelatedtoAssetsHeldforSale
Namespace Prefix:	slg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the increase (decrease) in rental revenue during the period resulting from the net amortization of acquired above-market and below-market leases.

+ References

No definition available.

+ Details

Name:	slg_RentalRevenueIncreaseDecreaseAcquiredLeaseAmortization
Namespace Prefix:	slg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, before tax, of (gain) loss recognized for the (reversal of write-down) write-down to fair value, less cost to sell, of a disposal group. Excludes discontinued operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=51719941&loc=d3e2941-110230

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=51824106&loc=d3e2443-110228

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

+ Details

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

- Definition

Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=26713463&loc=d3e16323-109275

+ Details

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

- Definition

Amount before amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

+ Details

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

- Definition

Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

+ Details

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

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_AboveMarketLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=slg_LeasesBelowMarketMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=slg_InPlaceLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=slg_A140150GrandStreetMember
Namespace Prefix:
Data Type:	na
Balance Type:
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Significant Accounting Policies (Details 2) $ in Thousands	3 Months Ended		9 Months Ended
Significant Accounting Policies (Details 2) $ in Thousands	Sep. 30, 2015 USD ($) property	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Investment in Commercial Real Estate Properties
Number of properties expected to be sold \| property	2
Increase in rental revenue from amortization of acquired leases	$ 10,200	$ 5,700	$ 32,900	$ 18,200
Reduction in interest expense from amortization of above-market rate mortgages	600	$ 1,300	1,700	$ 4,100
Identified intangible assets (included in other assets):
Gross amount	939,518		939,518		$ 664,297
Accumulated amortization	(392,854)		(392,854)		(383,236)
Net	546,664		546,664		281,061
Identified intangible liabilities (included in deferred revenue):
Gross amount	866,561		866,561		655,755
Accumulated amortization	(476,272)		(476,272)		(483,948)
Net	390,289		390,289		$ 171,807
Net intangible assets reclassed to assets held for sale	200		200
Net intangible liabilities reclassed to liabilities relates to assets held for sale	100		$ 100
Above-market leases \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			1 year
Above-market leases \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			14 years
Below-market leases \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			1 year
Below-market leases \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			14 years
In-place leases \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			1 year
In-place leases \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			14 years
Building \| Minimum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)			3 years
Building \| Maximum
Investment in Commercial Real Estate Properties
Estimated useful life (in years)			40 years
140-150 Grand Street
Investment in Commercial Real Estate Properties
Impairment Charge	$ 19,200