Property and Equipment, Net (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Office and computer equipment (3 - 10 year estimated useful life)	$ 219.5	$ 189.1
Office furniture and fixtures (3 - 10 year estimated useful life)	50.5	47.1
Internal-use computer software (1 - 10 year estimated useful life)	520.3	452.1
Leasehold improvements (1 - 20 year estimated useful life)	240.8	233.1
Total property and equipment, at cost	1,031.1	921.4
Less: accumulated depreciation and amortization	(706.0)	(595.5)
Total property and equipment, net	$ 325.1	$ 325.9