Customer Trust Fund - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investment income from invested customer trust fund included in revenue	$ 15.8	$ 11.6	$ 49.6	$ 34.2
Maturities ranging from one to 120 days [Member]
Average customer trust funds held in liquidity portfolios, Percentage			46.00%
Maturities ranging from 120 days to10 years [Member]
Average customer trust funds held in fixed income portfolios, Percentage			54.00%