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Schedule of Stock Option Aggregated Intrinsic Value Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award      
Exercised $ 8,035 $ 3,729 $ 7,255
Outstanding 14,831 12,468 11,729
Vested and expected to vest 14,831 12,468 11,729
Exercisable $ 9,076 $ 10,217 $ 10,716