Property and Equipment, Net - Components of Property and Equipment, Net (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 96.5	$ 106.9
Less: Accumulated depreciation	(70.2)	(67.6)
Total property and equipment, net	26.3	39.3
Computer Equipment [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	32.5	30.4
Software [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	35.5	35.1
Furniture and Fixtures [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	7.6	10.6
Leasehold Improvements [Member]
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 20.9	$ 30.8