Partners' Capital of the Operating Partnership - EPS (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
Numerator
Net income attributable to SL Green common stockholders (numerator for basic earnings per share)									$ 253,745	$ 231,760	$ 85,953
Add back: Dilutive effect of earnings allocated to participating securities									$ 1,719	$ 552	$ 471
Denominator
Weighted average common stock outstanding (in shares)									81,733,000	86,753,000	98,571,000
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
SL Green Operating Partnership
Numerator
Net income attributable to SLGOP common unitholders	$ 18,426	$ 34,876	$ 169,413	$ 46,070	$ (64,658)	$ 93,006	$ 109,142	$ 107,038	$ 268,785	$ 244,528	$ 90,419
Less: distributed earnings allocated to participating securities									(1,739)	(552)	(471)
Net income attributable to SL Green common stockholders (numerator for basic earnings per share)									267,046	243,976	89,948
Add back: Dilutive effect of earnings allocated to participating securities									1,719	$ 552	$ 471
Income attributable to SLGOP common unitholders									$ 268,765
Denominator
Weighted average common stock outstanding (in shares)									86,008,000	91,315,000	103,127,000
Stock-based compensation plans (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