Partners' Capital of the Operating Partnership - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 shares	Dec. 31, 2017 unit shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Common Stock
Stockholders' Equity
Units outstanding (units)	83,684,000	92,803,000	100,562,000	99,976,000
SL Green Operating Partnership
Stockholders' Equity
Noncontrolling interest in the operating partnership (as a percent)	4.70%	4.58%
Number of units of operating partnership owned by the noncontrolling interest unit holders (units)	4,130,579	4,452,979
SL Green Operating Partnership | Series I Preferred Units
Stockholders' Equity
Units outstanding (units)	9,200,000
Common units redemption, period of restriction	1 year
Conversion of stock, shares issued (in shares)	1
SL Green Operating Partnership | Common Stock
Stockholders' Equity
Units outstanding (units)	83,683,847