Debt - Schedule of Net Carrying Amount of Liability and Equity Components of Notes (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Dec. 17, 2020	Jun. 14, 2019
Liability component:
Principal	$ 1,564.5	$ 1,564.5
Unamortized debt discount	(213.4)	(249.9)
Unamortized transaction costs	(20.9)	(24.7)
Net carrying amount	1,330.2	1,289.9
Equity component, net of transaction costs	278.6	278.6
Convertible Senior Notes Due 2024 [Member]
Liability component:
Principal	690.0	690.0		$ 690.0
Unamortized debt discount	(66.4)	(83.8)
Unamortized transaction costs	(8.3)	(10.5)
Net carrying amount	615.3	595.7
Equity component, net of transaction costs	114.9	114.9		$ 118.0
Convertible Senior Notes Due 2026 [Member]
Liability component:
Principal	874.5	874.5	$ 874.5
Unamortized debt discount	(147.0)	(166.1)
Unamortized transaction costs	(12.6)	(14.2)
Net carrying amount	714.9	694.2
Equity component, net of transaction costs	$ 163.7	$ 163.7	$ 167.1