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Convertible Senior Notes - Debt Net of Discounts and Deferred Financing Costs (Detail) - USD ($)
$ in Millions
Sep. 30, 2020
Dec. 31, 2019
Debt Disclosure [Abstract]    
Principal $ 517.5 $ 517.5
Deferred financing costs (5.9) (6.9)
Debt discount, net (86.6) (101.8)
Net carrying amount $ 425.0 $ 408.8