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Debt - Carrying Amount of Liability Component of Notes (Detail)
$ in Millions
Mar. 31, 2021
USD ($)
Debt Instrument [Line Items]  
Principal amount $ 1,238.0
Convertible Senior Notes [Member]  
Debt Instrument [Line Items]  
Principal amount 575.0
Convertible Senior Notes [Member] | Liability Component [Member]  
Debt Instrument [Line Items]  
Principal amount 575.0
Less:  
Less unamortized discount on Term Debt and Convertible Senior Notes 107.9
Unamortized debt issuance costs 11.5
Net carrying amount $ 455.6