Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic Earnings:
Income attributable to SL Green common stockholders	$ 88,209	$ 38,869	$ 293,531	$ 58,442
Net Income (Loss) Available To Common Stockholders, After Distributed And Undistributed Earnings Allocation, Basic	88,051	38,760	293,022	58,112
Effect of Dilutive Securities:
Add back: undistributed earnings allocated to participating securities	33	0	138	0
Add back: distributed earnings allocated to participating securities	125	109	371	330
Diluted Earnings:
Income attributable to SL Green common stockholders	$ 93,006	$ 40,681	$ 309,187	$ 61,149
Basic Shares:
Weighted average common shares outstanding (shares)	85,566,000	97,783,000	87,692,000	99,431,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (shares)	4,643,000	4,543,000	4,677,000	4,570,000
Stock-based compensation plans (shares)	219,000	244,000	211,000	279,000
Diluted weighted average common stock 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		1,137,971
Common Stock
Effect of Dilutive Securities:
Redemption of units to common shares	$ 4,797	$ 1,812	$ 15,656	$ 2,707