Commitments and Contingencies - Components of Lease Cost, Supplemental Cash Flow and Other Information Related to Leases (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lease, Cost [Abstract]
Operating lease cost	$ 32.5	$ 32.5
Sublease income	(1.0)	(1.0)
Total lease cost	31.5	31.5
Operating cash flows from operating leases	28.5	26.8
Right-of-use assets obtained in exchange for lease obligations	$ 13.6	$ 91.1
Weighted-average remaining lease term of operating leases (years)	4 years 8 months 12 days	5 years 2 months 12 days
Weighted-average discount rate of operating leases	5.50%	6.30%