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

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

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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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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

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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

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=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

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

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- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_AboveMarketLeasesMember
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:
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- Details

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Significant Accounting Policies - Investments in Commercial Real Estate Properties (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investment in Commercial Real Estate Properties
Depreciable real estate reserves	$ 0	$ (29,064)	$ 0	$ (85,336)
Rental revenue from amortization of acquired leases	1,800	6,400	4,200	11,200
Reduction in interest expense from amortization of above-market rate mortgages	0	$ 700	0	$ 1,300
Identified intangible assets (included in other assets):
Gross amount	263,138		263,138		$ 325,880
Accumulated amortization	(239,344)		(239,344)		(277,038)
Net	23,794		23,794		48,842
Identified intangible liabilities (included in deferred revenue):
Gross amount	274,120		274,120		540,283
Accumulated amortization	(249,905)		(249,905)		(402,583)
Net	24,215		24,215		137,700
Net intangible assets reclassed to assets held for sale	100		100		13,900
Net intangible liabilities reclassed to liabilities relates to assets held for sale	$ 100		$ 100		$ 4,100
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