Partners' Capital of the Operating Partnership - EPS (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator
Net Income (Loss) Available To Common Stockholders, After Distributed And Undistributed Earnings Allocation, Basic	$ 88,051	$ 38,760	$ 293,022	$ 58,112
Add back: undistributed earnings allocated to participating securities	33	0	138	0
Add back: distributed earnings allocated to participating securities	125	109	371	330
Income attributable to SL Green common stockholders	$ 93,006	$ 40,681	$ 309,187	$ 61,149
Denominator
Weighted average common shares outstanding (shares)	85,566,000	97,783,000	87,692,000	99,431,000
Common stock shares excluded from the diluted shares outstanding (shares)	941,636		1,137,971
SL Green Operating Partnership
Stockholders' Equity
Basic weighted average common units outstanding (in shares)	90,209,000
Numerator
Net income attributable to SLGOP common unitholders	$ 93,006	$ 40,681	$ 309,187	$ 61,149
Less: distributed earnings allocated to participating securities	(125)	(109)	(371)	(330)
Less: undistributed earnings allocated to participating securities	(33)	0	(138)	0
Net Income (Loss) Available To Common Stockholders, After Distributed And Undistributed Earnings Allocation, Basic	92,848	40,572	308,678	60,819
Add back: undistributed earnings allocated to participating securities	33	0	138	0
Add back: distributed earnings allocated to participating securities	125	109	371	330
Income attributable to SL Green common stockholders	$ 93,006	$ 40,681	$ 309,187	$ 61,149
Denominator
Weighted average common shares outstanding (shares)		102,326,000	92,369,000	104,001,000
Stock-based compensation plans (shares)	219,000	244,000	211,000	279,000
Diluted weighted average common units outstanding (shares)	90,428,000	102,570,000	92,580,000	104,280,000
Common stock shares excluded from the diluted shares outstanding (shares)	941,636	1,175,708	1,137,971	1,076,695