Noncontrolling Interests on the Company's Consolidated Financial Statements - Common Unit Activity (Details) - USD ($) $ in Thousands	6 Months Ended	9 Months Ended	12 Months Ended
	Jun. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Rollforward Analysis of Preferred Unit Activity
Redemption of preferred units	$ (1,536)
SL Green Operating Partnership
Rollforward Analysis of Preferred Unit Activity
Redemption of preferred units	(1,536)
SL Green Operating Partnership | Preferred Units
Rollforward Analysis of Preferred Unit Activity
Balance at beginning of period	$ 283,285	$ 202,169	$ 283,285
Issuance of preferred units		0	0
Redemption of preferred units		(3,631)	(82,750)
Dividends paid on preferred units		(5,077)	(6,163)
Accrued dividends on preferred units		5,042	7,797
Balance at end of period		$ 198,503	$ 202,169