Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic Earnings:
Income attributable to SL Green common stockholders	$ 17,432	$ 33,157	$ 161,103	$ 43,792	$ (61,219)	$ 88,209	$ 103,556	$ 101,766	$ 255,484	$ 232,312	$ 86,424
Net income attributable to SL Green common stockholders (numerator for basic earnings per share)									253,745	231,760	85,953
Effect of Dilutive Securities:
Add back: Dilutive effect of earnings allocated to participating securities									1,719	552	471
Add back: Effect of dilutive securities (redemption of units to common shares)									13,301	12,216	3,995
Diluted Earnings:
Income attributable to SL Green common stockholders (numerator for diluted earnings per share)									$ 268,765	$ 244,528	$ 90,419
Basic Shares:
Weighted average common stock outstanding (in shares)									81,733,000	86,753,000	98,571,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (in shares)									4,275,000	4,562,000	4,556,000
Stock-based compensation plans (in shares)									554,000	215,000	276,000
Diluted weighted average common stock outstanding (in shares)									86,562,000	91,530,000	103,403,000
Common stock shares excluded from the diluted shares outstanding (shares)									1,252,670	1,138,647	774,782