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Stock-Based Compensation - Options Outstanding and Exercisable (Parenthetical) (Detail)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Options outstanding, aggregate intrinsic value $ 118.4
Options exercisable, weighted-average remaining contractual life 4 years 10 months 25 days
Options exercisable, aggregate intrinsic value $ 102.5