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Stockholders' Equity - Sanofi Funding (Details) - USD ($) $ in Millions | 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 | Sep. 30, 2019
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |
Treasury stock, shares acquired (in shares) | 9,806,805 |  |  |  |  |  |  |
Total cost of shares received | $ 5,000.0 | $ 100.4 | $ 5,071.8 | $ 336.0 | $ 48.7 | $ 54.0 |  |
Libtayo development |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |
Treasury stock, shares acquired (in shares) |  | 0 |  |  | 103,761 |  | 77,677 | 210,733
Total cost of shares received |  | $ 0.0 |  |  | $ 29.2 |  | $ 41.7 | $ 73.3
Dupilumab/REGN3500 Eligible Investments |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |
Treasury stock, shares acquired (in shares) |  | 0 |  |  | 69,143 |  | 171,471 | 93,286
Total cost of shares received |  | $ 0.0 |  |  | $ 19.4 |  | $ 93.3 | $ 29.4

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