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Customer Trust Fund - Amortized Cost and Fair Value of Investment Security Available for Sale (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Schedule of available-for-sale securities and cost-method investments [Line Items]    
Due in one year or less, Cost $ 2,339.5  
Due in one to three years, Cost 475.5  
Due in three to five years, Cost 588.1  
Due after five years, Cost 319.1  
Invested customer trust funds, Cost 3,722.2 $ 4,097.2
Due in one year or less, Fair Value 2,339.3  
Due in one to three years, Fair Value 469.9  
Due in three to five years, Fair Value 580.8  
Due after five years, Fair Value 307.8  
Invested customer trust funds, Fair Value $ 3,697.8 $ 4,091.4