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Debt - Components of Interest Expense and Effective Interest Rates of Senior Convertible Notes (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Cash interest expense:        
Contractual coupon interest $ 2.4 $ 2.4 $ 4.7 $ 4.7
Non-cash interest expense:        
Accretion of debt discount 16.5 11.3 28.3 22.6
Amortization of debt issuance costs 1.1 0.9 2.0 1.8
Total interest expense recognized on senior notes $ 20.0 $ 14.6 $ 35.0 $ 29.1
Senior Convertible Notes due 2022        
Non-cash interest expense:        
Effective interest rate (as a percent) 5.10% 5.10% 5.10% 5.10%
Senior Convertible Notes due 2023        
Non-cash interest expense:        
Effective interest rate (as a percent) 5.60% 5.60% 5.60% 5.60%
Senior Convertible Notes due 2025        
Non-cash interest expense:        
Effective interest rate (as a percent) 5.50%   5.50%