Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Basic Earnings:
Income attributable to SL Green common stockholders	$ 388,205	$ 13,859	$ 486,073	$ 185,104
Less: distributed earnings allocated to participating securities	(363)	(333)	(1,090)	(777)
Less: undistributed earnings allocated to participating securities	(1,942)	0	(1,736)	(141)
Net Income attributable to common unitholders (numerator for basic earnings per unit)	385,900	13,526	483,247	184,186
Effect of Dilutive Securities:
Add back: dilutive effect of earnings allocated to participating securities	363	333	1,090	777
Add back: undistributed earnings allocated to participating securities	1,942	0	1,736	141
Redemption of units to common shares	22,683	802	28,489	10,073
Diluted Earnings:
Income attributable to SL Green common stockholders	$ 410,888	$ 14,661	$ 514,562	$ 195,177
Basic Shares:
Weighted average common shares outstanding (shares)	66,840,000	70,943,000	68,275,000	73,373,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (shares)	3,834,000	4,029,000	4,024,000	4,123,000
Stock-based compensation plans (shares)	813,000	442,000	684,000	441,000
Contingently issuable shares from special dividend declared December 4, 2020 (in shares)	0	0	104,000	0
Diluted weighted average common stock outstanding (shares)	71,487,000	75,414,000	73,087,000	77,937,000
Common stock shares excluded from the diluted shares outstanding (shares)	969,631		1,032,243	1,728,767