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Customer Trust Fund - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Investment income from invested customer trust fund included in revenue $ 16.2 $ 11.2 $ 33.8 $ 22.6
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%