Represents the amount of flood coverage under each insurance policy held by the entity.

No definition available.

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

- Definition

Represents the grant date value based on the entity's stock price of deferred compensation awards granted under employment agreements.

+ References

No definition available.

+ Details

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

- Definition

Represents the minimum cash-based compensation, including base salary and guaranteed bonus payments, required in the next fiscal year under employment agreements.

+ References

No definition available.

+ Details

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

- Definition

Represents the maximum coverage per claim provided by the first property insurance portfolio.

+ References

No definition available.

+ Details

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

- Definition

Insurance Coverage, Per Annum Increase

+ References

No definition available.

+ Details

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

- Definition

Insurance Coverage, Program Trigger Amount

+ References

No definition available.

+ Details

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

- Definition

Represents the maximum coverage per claim and in aggregate per location provided by liability policies covering all properties.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of property insurance portfolios for which all-risk property and rental value coverage is maintained.

+ References

No definition available.

+ Details

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

- Definition

Represents the maximum coverage per claim provided by the second property insurance portfolio.

+ References

No definition available.

+ Details

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

- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 3

-Subparagraph (a),(b),(c)

-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437

+ Details

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

- Details

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

- Details

Name:	dei_LegalEntityAxis=slg_BelmontInsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=slg_PropertyAndRentalValueCoverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LossContingenciesByNatureOfContingencyAxis=slg_ResidentialAssetsCoverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=slg_TerrorismRiskInsuranceProgramReauthorizationandExtensionActof2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended	10 Months Ended
	Dec. 31, 2014	Oct. 31, 2012	Dec. 31, 2013
Commitments and Contingencies.
Employment agreements with certain executives, minimum cash-based compensation for next fiscal year	$ 4.6
Employment agreements with certain executives, grant date value of deferred compensation awards	1.3
Number of property insurance portfolios for which all-risk property and rental value coverage is maintained (portfolio)	3slg_NumberOfPropertyInsurancePortfolios
TRIPRA 2015
Commitments and Contingencies.
Program trigger	100.0
Per annum increase	20
Belmont
Commitments and Contingencies.
Loss reserves	6.1		7.0
Property and rental value coverage
Commitments and Contingencies.
Blanket limit per occurrence maintained under first property portfolio (up to $950 million)	950.0
Limit per occurrence maintained under property portfolio	700.0
Amount of flood coverage (up to $100 million)	100.0
Limit per occurrence and in aggregate per location under liability policies (up to $201 million)		201.0
Residential assets coverage
Commitments and Contingencies.
Limit per occurrence maintained under property portfolio	$ 380.0