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Debt - Components of Interest Expense and Effective Interest Rates of Senior Convertible Notes (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Cash interest expense:      
Contractual coupon interest $ 9.3 $ 9.3 $ 3.5
Non-cash interest expense:      
Accretion of debt discount (68.6) (45.8) (16.0)
Amortization of debt issuance costs 4.0 3.7 1.8
Total interest expense recognized on senior notes $ 81.9 $ 58.8 $ 21.3
Convertible Notes due 2022      
Non-cash interest expense:      
Effective interest rate (as a percent) 5.10% 5.10% 5.10%
Convertible Notes due 2023      
Non-cash interest expense:      
Effective interest rate (as a percent) 5.60% 5.60% 5.60%
Convertible Notes due 2025      
Non-cash interest expense:      
Effective interest rate (as a percent) 5.50%