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Debt 0.75% Senior convertible notes due 2022 (Details) - USD ($)
Sep. 30, 2018
Dec. 31, 2017
Debt Disclosure [Abstract]    
Principal amount $ 400,000,000.0 $ 400,000,000
Unamortized debt discount (54,400,000) (64,400,000)
Unamortized debt issuance costs (6,800,000) (8,000,000)
Net carrying amount of senior convertible notes 338,800,000 327,600,000
Fair value of outstanding notes 618,800,000 381,300,000
Amount by which the notes' if-converted value exceeds their principal amount $ 207,000,000 $ 0