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Commitments and Contingencies - Contingent Consideration Liabilities (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration $ 51.1 $ 18.7 $ 1.0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Contingent consideration $ 65.7 $ 19.5