Stockholders' Equity of the Company - Earnings per Share (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic Earnings:
Income attributable to SL Green common stockholders	$ 103,556	$ 8,222	$ 205,322	$ 19,573
Diluted Earnings:
Income attributable to SL Green common stockholders	$ 109,142	$ 8,641	$ 216,180	$ 20,468
Basic Shares:
Weighted average common shares outstanding (shares)	87,176,000	99,900,000	88,772,000	100,268,000
Effect of Dilutive Securities:
Operating Partnership units redeemable for common shares (shares)	4,706,000	4,562,000	4,695,000	4,584,000
Stock-based compensation plans (shares)	201,000	270,000	200,000	288,000
Diluted weighted average common stock outstanding (shares)	92,083,000	104,732,000	93,667,000	105,140,000
Common stock shares excluded from the diluted shares outstanding (shares)	1,158,317		1,184,735
Common Stock
Effect of Dilutive Securities:
Redemption of units to common shares	$ 5,586	$ 419	$ 10,858	$ 895