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Debt 0.75% Senior convertible notes due 2022 (Details) - USD ($) $ in Millions | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Debt Disclosure [Abstract] |  |
Current Fiscal Year End Date | --12-31 |
Principal amount | $ 400.0 | $ 400.0
Unamortized debt discount | (57.8) | (64.4)
Unamortized debt issuance costs | (7.2) | (8.0)
Net carrying amount of senior convertible notes | 335.0 | 327.6
Fair value of outstanding notes | 465.8 | 381.3
Amount by which the notes' if-converted value exceeds their principal amount | $ 55.4 | $ 0.0

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