Represents the maximum percentage of concentration risk that is not individually disclosed.

No definition available.

Name:	slg_ConcentrationRiskPercentageThreshold
Namespace Prefix:	slg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of collective bargaining arrangements.

+ References

No definition available.

+ Details

Name:	slg_NumberOfCollectiveBargainingArrangements
Namespace Prefix:	slg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the number of tenants to whom the facility is leased.

+ References

No definition available.

+ Details

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

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

- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

+ Details

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=slg_AnnualizedRentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_CustomerConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	slg_RealEstateByLocationAxis=slg_A11MadisonAveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	slg_RealEstateByLocationAxis=slg_AvenueOfTheAmericas1185Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	slg_RealEstateByLocationAxis=slg_LexingtonAvenue420Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	slg_RealEstateByLocationAxis=slg_Broadway1515Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	slg_RealEstateByLocationAxis=slg_A1MadisonAveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	slg_RealEstateByLocationAxis=slg_East220Street42Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_WorkforceSubjectToCollectiveBargainingArrangementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByTypeAxis=us-gaap_LaborForceConcentrationRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=us-gaap_WorkforceSubjectToCollectiveBargainingArrangementsExpiringWithinOneYearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=slg_Tenant1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Significant Accounting Policies - Concentrations of Credit Risk/Reclassification (Details)	12 Months Ended
	Dec. 31, 2020 Tenant agreement	Dec. 31, 2019	Dec. 31, 2018
Annualized rent \| Customer concentration
Concentration of Credit Risk
Number of tenants (tenants) \| Tenant	1
Maximum percentage of annualized rent for any one tenant not individually disclosed (percent) (more than)	5.00%
Annualized rent \| 11 Madison Avenue \| Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	8.20%	7.60%	6.50%
Annualized rent \| 1185 Avenue of the Americas \| Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	7.50%	7.40%	6.70%
Annualized rent \| 420 Lexington Avenue \| Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	6.90%	6.10%	6.00%
Annualized rent \| 1515 Broadway \| Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	6.60%	6.60%	7.40%
Annualized rent \| One Madison Ave \| Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	5.90%	6.00%	5.80%
Annualized rent \| East 220 Street 42 \| Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	5.40%	5.50%
Collective bargaining arrangements \| Workforce concentration
Concentration of Credit Risk
Percentage of concentration (percent)	64.10%
Number of collective bargaining agreements	6
Workforce Subject to Collective Bargaining Arrangements Expiring within One Year \| Workforce concentration
Concentration of Credit Risk
Number of collective bargaining agreements	0
Tenant 1 \| Annualized rent \| Customer concentration
Concentration of Credit Risk
Percentage of concentration (percent)	5.60%