Partners' Capital of the Operating Partnership - EPS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator
Net income attributable to SL Green common stockholders (numerator for basic earnings per share)	$ 354,335	$ 253,782	$ 231,270
Add back: Dilutive effect of earnings allocated to participating securities	$ 1,770	$ 1,702	$ 1,042
Denominator
Weighted average common stock outstanding (in shares)	72,552,000	79,415,000	84,090,000
Diluted weighted average common stock outstanding (in shares)	77,243,000	84,234,000	89,071,000
Common stock shares excluded from the diluted shares outstanding (shares)	1,728,136	1,217,153	1,106,363
SL Green Operating Partnership
Numerator
Net income attributable to SLGOP common unitholders	$ 376,121	$ 268,785	$ 244,528
Less: distributed earnings allocated to participating securities	(1,687)	(1,702)	(1,042)
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic	(83)	0	0
Net income attributable to SL Green common stockholders (numerator for basic earnings per share)	374,351	267,083	243,486
Add back: Dilutive effect of earnings allocated to participating securities	1,770	1,702	1,042
Income attributable to SLGOP common unitholders	$ 376,121	$ 268,785	$ 244,528
Denominator
Weighted average common stock outstanding (in shares)	76,647,000	83,690,000	88,652,000
Stock-based compensation plans (shares)	441,000	544,000	419,000
Contingently issuable units from special dividend declared December 4, 2020 (in shares)	155,000	0	0
Diluted weighted average common stock outstanding (in shares)	77,243,000	84,234,000	89,071,000
Common stock shares excluded from the diluted shares outstanding (shares)	1,728,136	1,217,153	1,106,363