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Schedule of Stock Options Weighted Average Remaining Contractual Terms (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award      
Outstanding 4 years 1 month 6 days 3 years 10 months 25 days 4 years 2 months 12 days
Vested and expected to vest 4 years 1 month 6 days 3 years 10 months 25 days 4 years 2 months 12 days
Exercisable 2 years 9 months 18 days 3 years 2 months 12 days 3 years 10 months 25 days