Customer Trust Fund - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Investment income from invested customer trust fund included in revenue	$ 17.6	$ 11.4
Maturities ranging from one to 120 days [Member]
Average customer trust funds held in liquidity portfolios, Percentage	45.00%
Maturities ranging from 120 days to10 years [Member]
Average customer trust funds held in fixed income portfolios, Percentage	55.00%