Partners' Capital of the Operating Partnership - Additional Information (Details) - shares	9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Common Stock
Stockholders' Equity
Units outstanding (units)	81,515,000	82,410,000	83,684,000	85,594,000	85,725,000	92,803,000
SL Green Operating Partnership
Stockholders' Equity
Noncontrolling interest in the operating partnership (as a percent)	4.96%		4.70%
Number of units of operating partnership owned by the noncontrolling interest unit holders (units)	4,257,754		4,130,579
SL Green Operating Partnership | Series I Preferred Units
Stockholders' Equity
Units outstanding (units)	9,200,000
Period of restriction to redeem OP Units	1 year
Conversion of stock, shares issued	1
SL Green Operating Partnership | Common Stock
Stockholders' Equity
Units outstanding (units)	81,515,066