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Fair Value Measurements - Asset and Liability Measured at Fair Value Measured on Recurring Basis (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Assets    
Available for sale customer trust funds assets $ 1,719.1 $ 1,826.8
Total assets measured at fair value 1,719.1 1,826.8
Fair Value, Inputs, Level 2 [Member]    
Assets    
Available for sale customer trust funds assets [1] 1,719.1 1,826.8
Total assets measured at fair value $ 1,719.1 $ 1,826.8
[1] Fair value is based on inputs that are observable for the asset or liability, other than quoted prices.