Partners' Capital of the Operating Partnership - EPS (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average common shares outstanding (shares)									98,571,000	100,185,000	99,345,000
Diluted weighted average common stock outstanding (shares)									103,403,000	104,881,000	103,734,000
Common stock shares excluded from the diluted shares outstanding (shares)									1,092,870	774,782	263,991
SL Green Operating Partnership
Net Income (Loss) Attributable to Parent [Abstract]
Net Income (Loss) Allocated to, Common Unitholders	$ 29,270	$ 40,681	$ 8,641	$ 11,827	$ 45,981	$ 35,915	$ 139,043	$ 24,143	$ 90,419	$ 245,082	$ 279,697
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Weighted average common shares outstanding (shares)									103,127,000	104,508,000	103,244,000
Stock-based compensation plans (shares)									276,000	373,000	490,000
Diluted weighted average common stock outstanding (shares)									103,403,000	104,881,000	103,734,000
Common stock shares excluded from the diluted shares outstanding (shares)									1,092,870	774,782	263,991