Customer Funds - Amortized Cost and Fair Value of Investment Security Available for Sale (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Cost	$ 3,000.7
Due in one to three years, Cost	678.7
Due in three to five years, Cost	403.9
Due after five years, Cost	138.0
Invested customer funds, Cost	4,221.3	$ 3,684.6
Due in one year or less, Fair Value	3,006.4
Due in one to three years, Fair Value	705.7
Due in three to five years, Fair Value	410.9
Due after five years, Fair Value	145.0
Invested customer funds, Fair Value	$ 4,268.0	$ 3,746.2