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Debt Schedule of senior convertible notes (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Nov. 30, 2018
Jun. 30, 2017
Senior Notes [Abstract]          
Net carrying amount of Senior Convertible Notes due 2022 $ 1,022,400,000   $ 1,010,300,000    
Senior Notes          
Senior Notes [Abstract]          
Fair value of outstanding senior convertible notes: 1,413,800,000   1,399,800,000    
Amount by which the notes' if-converted value exceeds their principal amount: 121,700,000 $ 125,400,000      
Senior Notes | Zero Point Seven Five Percent Senior Convertible Notes          
Senior Notes [Abstract]          
Total net carrying amount of senior convertible notes 1,022,400,000   1,010,300,000    
Senior Notes | Senior convertible notes due 2022          
Senior Notes [Abstract]          
Principal amount 400,000,000.0   400,000,000.0   $ 400,000,000.0
Unamortized debt discount (47,700,000)   (51,100,000)    
Unamortized debt issuance costs (5,900,000)   (6,300,000)    
Net carrying amount of Senior Convertible Notes due 2022 346,400,000   342,600,000    
Fair value of outstanding senior convertible notes: 537,500,000   540,200,000    
Amount by which the notes' if-converted value exceeds their principal amount: 121,700,000 125,400,000      
Senior Notes | Senior convertible notes due 2023          
Senior Notes [Abstract]          
Principal amount 850,000,000.0   850,000,000.0 $ 850,000,000  
Unamortized debt discount (164,000,000)   (171,800,000)    
Unamortized debt issuance costs (10,000,000)   (10,500,000)    
Net carrying amount of Senior Convertible Notes due 2022 676,000,000   667,700,000    
Fair value of outstanding senior convertible notes: 876,300,000   $ 859,600,000    
Amount by which the notes' if-converted value exceeds their principal amount: $ 0 $ 0