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Fair Value Measurements - Fair Value of Senior Convertible Notes (Details) - Senior Notes - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fair value of outstanding senior convertible notes $ 3,155.6 $ 2,150.8
Convertible Notes due 2022    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fair value of outstanding senior convertible notes   890.8
Convertible Notes due 2023    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fair value of outstanding senior convertible notes 1,936.4 $ 1,260.0
Convertible Notes due 2025    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total fair value of outstanding senior convertible notes $ 1,219.2