Partners' Capital of the Operating Partnership - Additional Information (Details)		12 Months Ended
		Dec. 31, 2020 shares	Dec. 31, 2019 unit shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares
Common Stock
Stockholders' Equity
Units, outstanding	[1]	68,508,000	76,956,000	81,311,000	90,172,000
SL Green Operating Partnership
Stockholders' Equity
Number of units of operating partnership owned by the noncontrolling interest unit holders (units)		3,938,823	4,195,875
SL Green Operating Partnership | Series I Preferred Units
Stockholders' Equity
Units, outstanding		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		68,508,127

[1] On January 21, 2021, we completed a reverse stock split whereby every 1.02918 SL Green common share was combined into 1 SL Green common share. We have retroactively adjusted the outstanding share counts, share activity, cash distributions declared, and earnings per share, as if the reverse split occurred on December 31, 2017.