Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic Earnings:
Income attributable to SL Green common stockholders	$ 356,105	$ 255,484	$ 232,312
Net income attributable to SL Green common stockholders (numerator for basic earnings per share)	354,335	253,782	231,270
Effect of Dilutive Securities:
Add back: Dilutive effect of earnings allocated to participating securities	1,770	1,702	1,042
Add back: Effect of dilutive securities (redemption of units to common shares)	20,016	13,301	12,216
Diluted Earnings:
Income attributable to SL Green common stockholders (numerator for diluted earnings per share)	$ 376,121	$ 268,785	$ 244,528
Basic Shares:
Weighted average common stock outstanding (in shares)	72,552,000	79,415,000	84,090,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (in shares)	4,096,000	4,275,000	4,562,000
Stock-based compensation plans (in shares)	440,000	544,000	419,000
Contingently issuable shares 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