Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic Earnings:
Income attributable to SL Green common stockholders	$ 56,444	$ 161,103	$ 171,245	$ 204,895
Less: distributed earnings allocated to participating securities	(333)	(128)	(444)	(257)
Less: undistributed earnings allocated to participating securities	0	(164)	(391)	(115)
Net Income attributable to common unitholders (numerator for basic earnings per unit)	56,111	160,811	170,410	204,523
Effect of Dilutive Securities:
Add back: dilutive effect of earnings allocated to participating securities	333	128	444	257
Add back: undistributed earnings allocated to participating securities	0	164	391	115
Redemption of units to common shares	3,070	8,310	9,272	10,587
Diluted Earnings:
Income attributable to SL Green common stockholders	$ 59,514	$ 169,413	$ 180,517	$ 215,482
Basic Shares:
Weighted average common shares outstanding (shares)	75,690,000	82,971,000	76,782,000	83,141,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (shares)	4,120,000	4,260,000	4,170,000	4,296,000
Stock-based compensation plans (shares)	409,000	167,000	440,000	169,000
Diluted weighted average common stock outstanding (shares)	80,219,000	87,398,000	81,392,000	87,606,000
Common stock shares excluded from the diluted shares outstanding (shares)	2,192,240		1,643,616

Distributed Earnings, Diluted

No definition available.

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

- Definition

Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of the redemption of units to common shares.

+ References

No definition available.

+ Details

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

- Definition

Net Income (Loss) Available To Common Stockholders, After Distributed And Undistributed Earnings Allocation, Basic

+ References

No definition available.

+ Details

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

- Definition

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of increase (decrease) to net income used for calculating diluted earnings per share (EPS), resulting from the assumed exercise of dilutive convertible securities excluding adjustments related to ESOP convertible preferred stock, stock options, and restrictive stock units.

+ References

No definition available.

+ Details

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

- Definition

The total amount of dividends declared in the period for each class of stock and the contractual amount of dividends (or interest on participating income bonds) that must be paid for the period (for example, unpaid cumulative dividends). Dividends declared in the current period do not include dividends declared in respect of prior-period unpaid cumulative dividends. Preferred dividends that are cumulative only if earned are deducted only to the extent that they are earned.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 66
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e2814-109256

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-URI http://asc.fasb.org/extlink&oid=117326831&loc=SL5780133-109256

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e2740-109256

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60A
-URI http://asc.fasb.org/extlink&oid=117326831&loc=SL5780132-109256

+ Details

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

- Definition

Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 28A
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e1500-109256

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

+ Details

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

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22694-107794

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e1377-109256

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities, and addition from assumption of issuance of common shares for dilutive potential common shares; of income (loss) available to common shareholders.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e1505-109256

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e1377-109256

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 60B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117326831&loc=SL5780133-109256

+ Details

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

- Definition

Amount of undistributed earnings (loss) allocated to participating securities for the diluted earnings (loss) per share or per unit calculation under the two-class method.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e1505-109256

+ Details

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

- Definition

Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=117326831&loc=d3e1448-109256

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

+ Details

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

- References

No definition available.

+ Details

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