Customer Funds - Amortized Cost and Fair Value of Investment Security Available for Sale (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Cost	$ 1,923.5
Due in one to three years, Cost	688.6
Due in three to five years, Cost	402.8
Due after five years, Cost	145.4
Invested customer funds, Cost	3,160.3	$ 3,684.6
Due in one year or less, Fair Value	1,931.6
Due in one to three years, Fair Value	712.4
Due in three to five years, Fair Value	407.5
Due after five years, Fair Value	152.3
Invested customer funds, Fair Value	$ 3,203.8	$ 3,746.2