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Customer Trust Fund - Amortized Cost and Fair Value of Investment Security Available for Sale (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]    
Due in one year or less, Cost $ 2,487.7  
Due in one to three years, Cost 753.4  
Due in three to five years, Cost 335.1  
Due after five years, Cost 108.4  
Invested customer trust funds, Cost 3,684.6 $ 3,153.2
Due in one year or less, Fair Value 2,491.2  
Due in one to three years, Fair Value 785.8  
Due in three to five years, Fair Value 349.1  
Due after five years, Fair Value 120.1  
Invested customer trust funds, Fair Value $ 3,746.2 $ 3,174.9