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Stockholders' Equity of the Company - Additional Information (Details) | 9 Months Ended | 12 Months Ended |  |  |  |  |
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Sep. 30, 2020 USD ($) increase $ / shares shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Jun. 30, 2020 shares | Sep. 30, 2019 shares | Jun. 30, 2019 shares | Jun. 30, 2018 USD ($) | Aug. 31, 2016 USD ($)
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Authorized capital stock (shares) | 260,000,000 |  |  |  |  |  |  |  |
Authorized shares, par value (in dollars per share) | $ / shares | $ 0.01 |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) | 160,000,000 | 160,000,000 |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares | $ 0.01 | $ 0.01 |  |  |  |  |  |  |
Excess stock, shares authorized (shares) | 75,000,000 |  |  |  |  |  |  |  |
Excess stock, par value (in dollars per share) | $ / shares | $ 0.01 |  |  |  |  |  |  |  |
Preferred stock, shares authorized (shares) | 25,000,000 |  |  |  |  |  |  |  |
Preferred stock, par value (in dollars per share) | $ / shares | $ 0.01 |  |  |  |  |  |  |  |
Excess shares issued (shares) | 0 |  |  |  |  |  |  |  |
Share repurchased (in shares) | 6,243,165 | 4,596,171 | 9,744,911 | 8,342,411 |  |  |  |  |
Average price paid per share (in dollars per share) | $ / shares | $ 62.77 | $ 83.62 | $ 96.22 | $ 101.64 |  |  |  |  |
2016 Repurchase Program |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Stock repurchase program, authorized amount | $ |  | $ 500,000,000.0 | $ 500,000,000.0 | $ 500,000,000.0 |  |  |  | $ 500,000,000.0 | $ 1,000,000,000.0
Number of increases to share repurchase program | increase | 4 |  |  |  |  |  |  |  |
Share repurchased (in shares) | 28,926,658 | 22,683,493 | 18,087,322 | 8,342,411 |  |  |  |  |
Stock repurchase program, authorized amount | $ | $ 3,000,000,000.0 |  |  |  |  |  |  |  |
Settled In July 2020 |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Share repurchased (in shares) | 216,101 |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Shares, outstanding (in shares) | 73,040,000 | 79,202,000 | 83,684,000 |  | 73,675,000 | 81,515,000 | 82,410,000 |  |
SL Green Operating Partnership | Common Stock |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Shares, outstanding (in shares) | 73,040,291 |  |  |  |  |  |  |  |

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