Debt - Schedule of Net Carrying Amount of Liability and Equity Components of Notes (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Dec. 17, 2020	Jun. 14, 2019
Liability component:
Principal	$ 1,564.5	$ 1,564.5
Unamortized debt discount	(237.8)	(249.9)
Unamortized transaction costs	(23.5)	(24.7)
Net carrying amount	1,303.2	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	(78.1)	(83.8)
Unamortized transaction costs	(9.8)	(10.5)
Net carrying amount	602.1	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	(159.7)	(166.1)
Unamortized transaction costs	(13.7)	(14.2)
Net carrying amount	701.1	694.2
Equity component, net of transaction costs	$ 163.7	$ 163.7	$ 167.1

The carrying amount of the equity component of convertible debt which may be settled in cash upon conversion.

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Face (par) amount of debt instrument at time of issuance.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Liability component.

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