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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, 2019
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Accretion of debt discount $ 49.6 $ 17.9 $ 9.4
Total interest expense recognized on senior notes 58.8 21.3 11.1
Senior convertible notes due 2022      
Debt Instrument [Line Items]      
Contractual coupon interest 3.0 3.0 1.9
Accretion of debt discount 13.9 13.4 8.2
Amortization of debt issuance costs 1.7 1.6 1.0
Total interest expense recognized on senior notes $ 18.6 $ 18.0 $ 11.1
Effective interest rate (as a percent) 5.10% 5.10% 5.10%
Senior convertible notes due 2023      
Debt Instrument [Line Items]      
Contractual coupon interest $ 6.3 $ 0.5 $ 0.0
Accretion of debt discount 31.9 2.6 0.0
Amortization of debt issuance costs 2.0 0.2 0.0
Total interest expense recognized on senior notes $ 40.2 $ 3.3 $ 0.0
Effective interest rate (as a percent) 5.60% 5.60% 0.00%