Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Basic Earnings:
Income attributable to SL Green common stockholders	$ 105,332	$ 56,444	$ 97,868	$ 171,245
Less: distributed earnings allocated to participating securities	(369)	(333)	(738)	(444)
Less: undistributed earnings allocated to participating securities	(246)	0	0	(391)
Net Income attributable to common unitholders (numerator for basic earnings per unit)	104,717	56,111	97,130	170,410
Effect of Dilutive Securities:
Add back: dilutive effect of earnings allocated to participating securities	369	333	738	444
Add back: undistributed earnings allocated to participating securities	246	0	0	391
Redemption of units to common shares	6,282	3,070	5,806	9,272
Diluted Earnings:
Income attributable to SL Green common stockholders	$ 111,614	$ 59,514	$ 103,674	$ 180,517
Basic Shares:
Weighted average common shares outstanding (shares)	68,980,000	73,538,000	68,996,000	74,598,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (shares)	4,093,000	4,120,000	4,121,000	4,170,000
Stock-based compensation plans (shares)	654,000	408,000	625,000	440,000
Contingently issuable shares from special dividend declared December 4, 2020 (in shares)	0	0	157,000	0
Diluted weighted average common stock outstanding (shares)	73,727,000	78,066,000	73,899,000	79,208,000
Common stock shares excluded from the diluted shares outstanding (shares)	993,847		1,064,788	1,597,015