Stockholders' Equity of the Company - Additional Information (Details)	Jan. 15, 2021 $ / shares	Jan. 08, 2021	Sep. 30, 2021 $ / shares shares	Jun. 30, 2021 shares	Jan. 07, 2021 $ / shares	Dec. 31, 2020 $ / shares shares	Sep. 30, 2020 shares	Jun. 30, 2020 shares	Dec. 31, 2019 shares
Equity, Class of Treasury Stock [Line Items]
Authorized capital stock (shares) | shares			260,000,000
Authorized shares, par value (in dollars per share)			$ 0.01
Common stock, shares authorized (in shares) | shares			160,000,000			160,000,000
Common stock, par value (in dollars per share)			$ 0.01			$ 0.01
Excess stock, shares authorized (shares) | shares			75,000,000
Excess stock, par value (in dollars per share)			$ 0.01
Preferred stock, shares authorized (shares) | shares			25,000,000
Preferred stock, par value (in dollars per share)			$ 0.01
Excess shares issued (shares) | shares			0
Common Stock
Equity, Class of Treasury Stock [Line Items]
Shares, outstanding (in shares) | shares			66,865,000	67,880,000		68,508,000	70,969,000	71,586,000	76,956,000
Share price (in dollars per share)					$ 58.15
Reverse stock split conversion ratio		1.02918
Common Stock | Cash Dividend
Equity, Class of Treasury Stock [Line Items]
Common stock, dividends, per share, cash paid (in dollars per share)	$ 0.3735
Common stock, dividends, per share, stock paid (in dollars per share)	0.0279
Common Stock | Stock Dividend
Equity, Class of Treasury Stock [Line Items]
Common stock, dividends, per share, stock paid (in dollars per share)	$ 0.0343
SL Green Operating Partnership | Common Stock
Equity, Class of Treasury Stock [Line Items]
Shares, outstanding (in shares) | shares			66,865,416