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Fair Value Measurements Fair Value of Convertible Senior Notes (Details) - Senior Notes - USD ($) $ in Millions | Sep. 30, 2019 | Dec. 31, 2018
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Fair Value Disclosures [Line Items] |  |
Fair value of outstanding senior convertible notes: | $ 1,632.1 | $ 1,399.8
Senior convertible notes due 2022 |  |
Fair Value Disclosures [Line Items] |  |
Fair value of outstanding senior convertible notes: | 635.6 | 540.2
Senior convertible notes due 2023 |  |
Fair Value Disclosures [Line Items] |  |
Fair value of outstanding senior convertible notes: | $ 996.5 | $ 859.6

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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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