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Stock-Based Compensation Plans and Employee Benefits - Summary of Activity of Employee Stock Option Exercises (Detail) - Stock options [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Cash received for exercise price $ 0.4 $ 9.9 $ 2.5
Intrinsic value $ 0.2 $ 6.0 $ 1.2