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Customer Trust Fund - Amortized Cost and Fair Value of Investment Security Available for Sale (Detail) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]      
Due in one year or less, Cost $ 2,024.4 $ 2,761.6  
Due in one to three years, Cost 565.6 518.3  
Due in three to five years, Cost 565.5 528.8  
Due after five years, Cost 292.9 288.5  
Invested customer trust funds, Cost 3,448.4 4,097.2 $ 3,686.8
Due in one year or less, Fair Value 2,023.9 2,762.0  
Due in one to three years, Fair Value 556.5 648.8  
Due in three to five years, Fair Value 557.6 473.9  
Due after five years, Fair Value 280.0 206.7  
Invested customer trust funds, Fair Value $ 3,418.0 $ 4,091.4 $ 3,697.3