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Debt - Conversion Value of Convertible Notes (Details) - Senior Notes - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Total by which the notes’ if-converted value exceeds their principal amount $ 1,285.0 $ 858.6
Senior convertible notes due 2022    
Debt Instrument [Line Items]    
Total by which the notes’ if-converted value exceeds their principal amount 686.9 486.2
Senior convertible notes due 2023    
Debt Instrument [Line Items]    
Total by which the notes’ if-converted value exceeds their principal amount $ 598.1 $ 372.4