Customer Trust Fund - Amortized Cost and Fair Value of Investment Security Available for Sale (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Cost	$ 2,221.1
Due in one to three years, Cost	711.9
Due in three to five years, Cost	494.5
Due after five years, Cost	161.1
Invested customer trust funds, Cost	3,588.6	$ 3,153.2
Due in one year or less, Fair Value	2,222.1
Due in one to three years, Fair Value	725.0
Due in three to five years, Fair Value	513.4
Due after five years, Fair Value	172.4
Invested customer trust funds, Fair Value	$ 3,632.9	$ 3,174.9