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Fair Value Measurements - Fair Value of Convertible Senior Notes (Details) - Senior Notes - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Fair Value Disclosures [Line Items]    
Fair value of outstanding senior convertible notes: $ 2,562.9 $ 2,150.8
0.75% Senior Convertible Notes due 2022    
Fair Value Disclosures [Line Items]    
Fair value of outstanding senior convertible notes: 1,088.9 890.8
0.75% Senior Convertible Notes due 2023    
Fair Value Disclosures [Line Items]    
Fair value of outstanding senior convertible notes: $ 1,474.0 $ 1,260.0