This element represent the net income (loss) in consolidation of joint venture interest.

No definition available.

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

- Definition

Value of stock issued during the period from a deferred compensation plan and stock award, net.

+ References

No definition available.

+ Details

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

- Definition

Number of shares issued during the period from a Deferred Compensation Plan and Stock Award.

+ References

No definition available.

+ Details

Name:	slg_DeferredCompensationPlanAndStockAwardShares
Namespace Prefix:	slg_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Distributions declared for preferred units during the period.

+ References

No definition available.

+ Details

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

- Definition

Other Comprehensive Income (Loss), Net of Tax, Less Portion Attributable to Noncontrolling Interest

+ References

No definition available.

+ Details

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

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest, less preferred distributions during the period.

+ References

No definition available.

+ Details

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

- Definition

Reallocation of Noncontrolling Interest To (From) Operating Partnership

+ References

No definition available.

+ Details

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

- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

+ Details

Name:	us-gaap_CommonStockDividendsPerShareDeclared
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Decrease in noncontrolling interest balance from payment of dividends or other distributions by the non-wholly owned subsidiary or partially owned entity, included in the consolidation of the parent entity, to the noncontrolling interest holders.

+ References

+ Details

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

- Definition

Decrease in noncontrolling interest (for example, but not limited to, redeeming or purchasing the interests of noncontrolling shareholders, issuance of shares (interests) by the non-wholly owned subsidiary to the parent entity for other than cash, and a buyback of shares (interest) by the non-wholly owned subsidiary from the noncontrolling interests).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 23
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569655-111683

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 24
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4616395-111683

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

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

- Definition

Total distributions to each class of partners (i.e., general, limited and preferred partners).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 5
-Subparagraph (SAB Topic 4.F)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770

+ Details

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

- Definition

The number of each class of partnership units outstanding at the balance sheet date. Units represent shares of ownership of the general, limited, and preferred partners.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04.16(a))
-URI http://asc.fasb.org/extlink&oid=6488278&loc=d3e603758-122996

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04.16(b))
-URI http://asc.fasb.org/extlink&oid=6488278&loc=d3e603758-122996

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 5
-Subparagraph (SAB TOPIC 4.F)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770

+ Details

Name:	us-gaap_PartnersCapitalAccountUnits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Total units issued during the year due to the sale of units. All partners include general, limited and preferred partners.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_PartnersCapitalAccountUnitsSaleOfUnits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of partners' capital (deficit), including portions attributable to both the parent and noncontrolling interests. Excludes temporary equity and is sometimes called permanent equity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 272
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6373374&loc=d3e70478-108055

+ Details

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

- Definition

Number of shares issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the shareholders to reinvest dividends paid to them by the entity on new issues of stock by the entity.

+ References

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Value of stock issued during the period upon the conversion of units. An example of a convertible unit is an umbrella partnership real estate investment trust unit (UPREIT unit).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-31)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

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

- Definition

Value of stock issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the holder of the stock to reinvest dividends paid to them by the entity on new issues of stock by the entity.

+ References

+ Details

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

- Definition

Number of shares that have been repurchased during the period and are being held in treasury.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_TreasuryStockSharesAcquired
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

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

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_CommonStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_NoncontrollingInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_SubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PartnerCapitalComponentsAxis=us-gaap_GeneralPartnerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=slg_SeriesIPreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_PreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	slg_CumulativeEffectPeriodOfAdoptionAxis=slg_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	slg_CumulativeEffectPeriodOfAdoptionAxis=slg_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Consolidated Statements of Capital (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2020	Dec. 31, 2019
Increase (Decrease) in Partner's Capital
Acquisition of subsidiary interest from noncontrolling interest	$ 0		$ (54)		$ (1,536)	$ (25,791)
DRSPP proceeds	202		39		364	263
Conversion of common units	0		25		8,744	446
Reallocation of noncontrolling interests in the operating partnership	4,109		(1,481)		31,144	(14,028)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	7,543		5,948		13,493	10,533
Repurchases of common stock	(31,127)		(74,531)		(360,868)	(109,313)
Contribution to consolidated joint venture interests	4,219		2,404		8,185	50,692
Cash distributions to noncontrolling interests	$ (12,530)		$ (207)		$ (596)	$ (271)
Cash distribution declared, per common share (in dollars per share)	$ 0.885	$ 1.180	$ 0.85	$ 1.70
Common Stock
Increase (Decrease) in Partner's Capital
DRSPP proceeds (in shares)	4				6	3
Conversion of common units	$ 0				$ 1
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	3		21		(33)	(17)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	$ 0
Repurchases of common stock (in shares)	(642)		(916)		(5,601)	(1,265)
Repurchases of common stock	$ (7)		$ (9)		$ (56)	$ (12)
Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Acquisition of subsidiary interest from noncontrolling interest	0				1,587	(25,276)
Contribution to consolidated joint venture interests	4,219		2,404		8,185	50,692
Cash distributions to noncontrolling interests	(12,530)		(207)		(596)	(271)
SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance	5,205,762		5,879,501		5,517,198	5,947,855	$ 5,517,198	$ 5,947,855
Net income (loss)	18,011		36,271		179,451	210,470
Acquisition of subsidiary interest from noncontrolling interest	0		(54)		(1,536)	(25,791)
Other comprehensive income	6,171		(11,737)		(53,886)	(43,503)
Preferred distributions	(3,738)		(3,738)		(7,475)	(7,475)
DRSPP proceeds	202		39		364	263
Conversion of common units	0		25		8,744	446
Reallocation of noncontrolling interests in the operating partnership	4,109		(1,481)		31,144	(14,028)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	7,543		5,948		13,493	10,533
Repurchases of common stock	(31,127)		(74,531)		(360,868)	(109,313)
Contribution to consolidated joint venture interests	4,219		2,404		8,185	50,692
Cash distributions to noncontrolling interests	$ (12,530)		$ (207)		(596)	(271)
Cash distribution declared, per common share (in dollars per share)	$ 0.885	$ 1.180	$ 0.85	$ 1.70
Cash distributions declared (per common unit, none of which represented a return of capital for federal income tax purposes)	$ (64,617)		$ (69,140)		(89,272)	(140,377)
Ending Balance	5,134,005	$ 5,205,762	5,763,300	$ 5,879,501	5,205,762	5,879,501	5,134,005	5,517,198
SL Green Operating Partnership \| Common Stock \| Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance	$ 4,980,411		$ 5,616,385		$ 5,247,868	$ 5,664,481	$ 5,247,868	$ 5,664,481
Beginning Balance (units)	73,675		82,410		79,202	83,684	79,202	83,684
Net income (loss)	$ 17,597		$ 36,895		$ 178,720	$ 212,370
Acquisition of subsidiary interest from noncontrolling interest	0		(54)		(3,123)	(515)
Preferred distributions	$ (3,738)		$ (3,738)		$ (7,475)	$ (7,475)
DRSPP proceeds (in shares)	4		0		6	3
DRSPP proceeds	$ 202		$ 39		$ 364	$ 263
Conversion of common units (in shares)	0		0		101	5
Conversion of common units	$ 0		$ 25		$ 8,744	$ 446
Reallocation of noncontrolling interests in the operating partnership	$ 4,109		$ (1,481)		$ 31,144	$ (14,028)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings (in shares)	3		21		(33)	(17)
Deferred compensation plan and stock awards, net of forfeitures and tax withholdings	$ 7,543		$ 5,948		$ 13,493	$ 10,533
Repurchases of common stock (in shares)	(642)		(916)		(5,601)	(1,265)
Repurchases of common stock	$ (31,127)		$ (74,531)		$ (360,868)	$ (109,313)
Cash distributions declared (per common unit, none of which represented a return of capital for federal income tax purposes)	(64,617)		(69,140)		(89,272)	(140,377)
Ending Balance	$ 4,910,380	$ 4,980,411	$ 5,510,348	$ 5,616,385	$ 4,980,411	$ 5,616,385	$ 4,910,380	$ 5,247,868
Ending Balance (units)	73,040	73,675	81,515	82,410	73,675	82,410	73,040	79,202
SL Green Operating Partnership \| Accumulated Other Comprehensive Loss
Increase (Decrease) in Partner's Capital
Beginning Balance	$ (82,371)		$ (28,395)		$ (28,485)	$ 15,108	$ (28,485)	$ 15,108
Other comprehensive income	6,171		(11,737)		(53,886)	(43,503)
Ending Balance	(76,200)	$ (82,371)	(40,132)	$ (28,395)	(82,371)	(28,395)	(76,200)	(28,485)
SL Green Operating Partnership \| Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance	85,790		69,579		75,883	46,334	75,883	46,334
Net income (loss)	414		(624)		731	(1,900)
Acquisition of subsidiary interest from noncontrolling interest	0				1,587	(25,276)
Contribution to consolidated joint venture interests	4,219		2,404		8,185	50,692
Cash distributions to noncontrolling interests	(12,530)		(207)		(596)	(271)
Ending Balance	77,893	85,790	71,152	69,579	85,790	69,579	77,893	75,883
Series I Preferred Stock \| SL Green Operating Partnership \| Preferred Units
Increase (Decrease) in Partner's Capital
Beginning Balance	221,932		221,932		221,932	221,932	221,932	221,932
Ending Balance	$ 221,932	$ 221,932	$ 221,932	$ 221,932	221,932	$ 221,932	221,932	221,932
Cumulative Effect, Period Of Adoption, Adjusted Balance \| SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance					5,478,014		5,478,014
Ending Balance								5,478,014
Cumulative Effect, Period Of Adoption, Adjusted Balance \| SL Green Operating Partnership \| Common Stock \| Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance					$ 5,208,684		$ 5,208,684
Beginning Balance (units)					79,202		79,202
Ending Balance								$ 5,208,684
Ending Balance (units)								79,202
Cumulative Effect, Period Of Adoption, Adjusted Balance \| SL Green Operating Partnership \| Accumulated Other Comprehensive Loss
Increase (Decrease) in Partner's Capital
Beginning Balance					$ (28,485)		$ (28,485)
Ending Balance								$ (28,485)
Cumulative Effect, Period Of Adoption, Adjusted Balance \| SL Green Operating Partnership \| Noncontrolling Interests
Increase (Decrease) in Partner's Capital
Beginning Balance					75,883		75,883
Ending Balance								75,883
Cumulative Effect, Period Of Adoption, Adjusted Balance \| Series I Preferred Stock \| SL Green Operating Partnership \| Preferred Units
Increase (Decrease) in Partner's Capital
Beginning Balance					221,932		221,932
Ending Balance								221,932
Accounting Standards Update 2016-13 \| Cumulative Effect, Period Of Adoption, Adjustment \| SL Green Operating Partnership
Increase (Decrease) in Partner's Capital
Beginning Balance					(39,184)		(39,184)
Ending Balance								(39,184)
Accounting Standards Update 2016-13 \| Cumulative Effect, Period Of Adoption, Adjustment \| SL Green Operating Partnership \| Common Stock \| Partners' Interest
Increase (Decrease) in Partner's Capital
Beginning Balance					$ (39,184)		$ (39,184)
Ending Balance								$ (39,184)