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Debt (Tables) | 9 Months Ended
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Sep. 30, 2018
Debt Disclosure [Abstract] |
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments | 0.75% Senior Convertible Notes due 2022
(in millions) | September 30, 2018 |  | December 31, 2017
0.75% Senior convertible notes due 2022: |  |  |
Principal amount | $ | 400.0 |  |  | $ | 400.0 |
Unamortized debt discount | (54.4 | ) |  | (64.4 | )
Unamortized debt issuance costs | (6.8 | ) |  | (8.0 | )
Net carrying amount of senior convertible notes | $ | 338.8 |  |  | $ | 327.6 |
Fair value of outstanding notes | $ | 618.8 |  |  | $ | 381.3 |
Amount by which the notes' if-converted value exceeds their principal amount | $ | 207.0 |  |  | $ | — |

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- Definition Tabular disclosure of information pertaining to carrying amount and estimated fair value of short-term and long-term debt instruments or arrangements, including but not limited to, identification of terms, features, and collateral requirements. + References No definition available. + Details
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