Noncontrolling Interests on the Company's Consolidated Financial Statements - Preferred Unit Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Rollforward Analysis of Preferred Unit Activity
Redemption of preferred units			$ (20,650)
SL Green Operating Partnership
Rollforward Analysis of Preferred Unit Activity
Issuance of preferred units	$ 25,723	$ 78,495
Redemption of preferred units	(21,574)	(31,805)
SL Green Operating Partnership | Preferred Units
Rollforward Analysis of Preferred Unit Activity
Balance at beginning of period	302,010	282,516
Issuance of preferred units	0	22,793
Redemption of preferred units	(275)	(3,299)
Balance at end of period	$ 301,735	$ 302,010	$ 282,516