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Benefit Plans - Summary of Activity of Employee Stock Option Exercises (Detail) - Stock options [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Cash received for exercise price $ 2.5 $ 1.5 $ 0.8
Intrinsic value $ 1.2 $ 1.1 $ 0.7