Depreciable Real Estate Reserves, Effect on Earnings

No definition available.

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

- Definition

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

+ References

No definition available.

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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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

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- 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
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- 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
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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

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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 of amortization of acquired leases acquired as part of a real property acquisition at below market lease rate with a finite life.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

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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=66006027&loc=d3e16323-109275

+ Details

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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

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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

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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

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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

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- Details

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Data Type:	na
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Period Type:

- Details

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

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- Details

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Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Investment in Commercial Real Estate Properties
Depreciable real estate reserves	$ 0	$ 0	$ (85,336,000)	$ (10,387,000)
Rental revenue from amortization of acquired leases	4,600,000	200,000	15,800,000	191,400,000
Reduction in interest expense from amortization of above-market rate mortgages	0	700,000	800,000	2,000,000
Identified intangible assets (included in other assets):
Gross amount	474,493,000		474,493,000		$ 651,099,000
Accumulated amortization	(398,047,000)		(398,047,000)		(410,930,000)
Net	76,446,000		76,446,000		240,169,000
Identified intangible liabilities (included in deferred revenue):
Gross amount	639,511,000		639,511,000		655,930,000
Accumulated amortization	(474,365,000)		(474,365,000)		(464,749,000)
Net	165,146,000		165,146,000		191,181,000
Net intangible assets reclassed to assets held for sale	3,900,000		3,900,000		0
Net intangible liabilities reclassed to liabilities relates to assets held for sale	1,100,000		$ 1,100,000		$ 0
Minimum \| Above-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			1 year
Minimum \| Below-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			1 year
Minimum \| In-place leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			1 year
Maximum \| Above-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			14 years
Maximum \| Below-market leases
Investment in Commercial Real Estate Properties
Estimated useful life of other intangible assets (in years)			14 years
Maximum \| In-place leases
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
388-390 Greenwich Street
Investment in Commercial Real Estate Properties
Amortization of below market lease		$ 0		$ 172,400,000
125 Chubb Avenue and Stamford Towers
Investment in Commercial Real Estate Properties
Depreciable real estate reserves	$ 0		$ 85,300,000