Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic Earnings:
Income attributable to SL Green common stockholders	$ 33,157	$ 88,209	$ 238,052	$ 293,531
Net Income (Loss) Available To Common Stockholders, After Distributed And Undistributed Earnings Allocation, Basic	33,049	88,051	237,686	293,022
Effect of Dilutive Securities:
Add back: distributed earnings allocated to participating securities	108	125	325	371
Add back: undistributed earnings allocated to participating securities	0	33	41	138
Diluted Earnings:
Income attributable to SL Green common stockholders	$ 34,876	$ 93,006	$ 250,358	$ 309,187
Basic Shares:
Weighted average common shares outstanding (shares)	82,292,000	85,566,000	82,855,000	87,692,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (shares)	4,258,000	4,643,000	4,283,000	4,677,000
Stock-based compensation plans (shares)	164,000	219,000	171,000	211,000
Diluted weighted average common stock outstanding (shares)	86,714,000	90,428,000	87,309,000	92,580,000
Common stock shares excluded from the diluted shares outstanding (shares)	1,317,803	941,636
Common Stock
Effect of Dilutive Securities:
Redemption of units to common shares	$ 1,719	$ 4,797	$ 12,306	$ 15,656