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Stockholders' Equity - Narrative (Details) - USD ($) |  | 3 Months Ended |
--- | --- | --- | ---
Dec. 31, 2020 | Mar. 31, 2021 | Mar. 31, 2020 | Jan. 31, 2021
Sanofi | Libtayo development |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of shares repurchased (in shares) |  | 43,627 |  |
Cost of treasury stock received |  | $ 21,400,000 |  |
Sanofi | Dupilumab/REGN3500 Eligible Investments |  |  |  |
Class of Stock [Line Items] |  |  |  |
Number of shares repurchased (in shares) |  | 85,287 |  |
Cost of treasury stock received |  | $ 41,800,000 |  |
2019 Share Repurchase Program |  |  |  |
Class of Stock [Line Items] |  |  |  |
Share repurchase program, authorized amount | $ 1,000,000,000.0 |  |  |
Number of shares repurchased (in shares) |  |  | 719,167 |
Cost of treasury stock received | $ 1,000,000,000.0 |  | $ 272,800,000 |
2021 Share Repurchase Program |  |  |  |
Class of Stock [Line Items] |  |  |  |
Share repurchase program, authorized amount |  |  |  | $ 1,500,000,000
Number of shares repurchased (in shares) |  | 690,265 |  |
Cost of treasury stock received |  | $ 323,500,000 |  |
Remaining authorized repurchase amount |  | $ 1,177,000,000 |  |

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