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Fair Value Measurements - Narrative (Details) - USD ($)
$ in Millions
Jun. 30, 2020
Dec. 31, 2019
Jun. 30, 2019
Senior Notes      
Debt Instrument [Line Items]      
Fair value $ 530.1 $ 587.5 $ 567.2
Convertible Senior Notes      
Debt Instrument [Line Items]      
Fair value $ 569.0