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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
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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 |  |  |  |  |  |

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