Noncontrolling Interests on the Company's Consolidated Financial Statements - Common Unit Activity (Details) - SL Green Operating Partnership - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Rollforward Analysis of Preferred Unit Activity
Issuance of preferred units	$ 19,980	$ 25,723
Redemption of preferred units	(12,005)	(21,574)
Preferred Units
Rollforward Analysis of Preferred Unit Activity
Balance at beginning of period	301,735	302,010
Issuance of preferred units	0	0
Redemption of preferred units	(450)	(275)
Balance at end of period	$ 301,285	$ 301,735