Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Basic Earnings:
Income attributable to SL Green common stockholders	$ (7,464)	$ 114,801
Less: distributed earnings allocated to participating securities	(371)	(113)
Less: undistributed earnings allocated to participating securities	0	(464)
Net Income attributable to common unitholders (numerator for basic earnings per unit)	(7,835)	114,224
Effect of Dilutive Securities:
Add back: dilutive effect of earnings allocated to participating securities	371	113
Add back: undistributed earnings allocated to participating securities	0	464
Redemption of units to common shares	(476)	6,202
Diluted Earnings:
Income attributable to SL Green common stockholders	$ (7,940)	$ 121,003
Basic Shares:
Weighted average common shares outstanding (shares)	69,010,000	75,656,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (shares)	4,148,000	4,220,000
Stock-based compensation plans (shares)	595,000	476,000
Contingently issuable shares from special dividend declared December 4, 2020 (in shares)	317,000	0
Diluted weighted average common stock outstanding (shares)	74,070,000	80,352,000
Common stock shares excluded from the diluted shares outstanding (shares)	1,159,343