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Stockholders' Equity - Narrative (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
May 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Sep. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2020 | Nov. 30, 2019
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Treasury stock, shares acquired (in shares) | 9,806,805 |  |  |  |  |  |  |
Cost of treasury stock received | $ 5,000,000,000 | $ 100,400,000 | $ 5,071,800,000 | $ 336,000,000.0 | $ 48,700,000 | $ 54,000,000.0 |  |
Sanofi Collaboration Agreement, Immuno-oncology |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Maximum shares the collaborator could sell (in shares) |  | 279,766 |  |  |  |  | 279,766 |
Sanofi |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Number of shares sold (in shares) | 13,014,646 |  |  |  |  |  |  |
Common Stock, shares outstanding (in shares) | 400,000 |  |  |  |  |  |  |
Share Repurchase Program |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Share repurchase program, authorized amount |  |  |  |  |  |  |  | $ 1,000,000,000.0
Remaining authorized repurchase amount |  | $ 372,700,000 |  |  |  |  | $ 372,700,000 |
Treasury stock, shares acquired (in shares) |  | 179,824 |  |  |  |  | 898,991 |
Cost of treasury stock received |  | $ 100,400,000 |  |  |  |  | $ 373,300,000 |

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