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Fair Value Measurements - Fair Value of Convertible Senior Notes (Details) - Senior Notes - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
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Fair Value Disclosures [Line Items] |  |
Fair value of outstanding senior convertible notes: | $ 4,151.2 | $ 3,155.6
Senior Convertible Notes due 2023 |  |
Fair Value Disclosures [Line Items] |  |
Fair value of outstanding senior convertible notes: | 2,719.0 | 1,936.4
Senior Convertible Notes due 2025 |  |
Fair Value Disclosures [Line Items] |  |
Fair value of outstanding senior convertible notes: | $ 1,432.2 | $ 1,219.2

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- Definition Fair value portion of borrowing which can be exchanged for a specified number of another security at the option of the issuer or the holder, for example, but not limited to, the entity's common stock. + References No definition available. + Details
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