Annual report pursuant to Section 13 and 15(d)

Stock-based Expense - Outstanding Stock Options, Vested and Expected to Vest, Non-Vested and Stock Options Currently Exercisable (Detail)

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Stock-based Expense - Outstanding Stock Options, Vested and Expected to Vest, Non-Vested and Stock Options Currently Exercisable (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]    
Number of options, options fully vested and expected to vest 2,200,465 3,606,462
Weighted average remaining contractual life (in years), options fully vested and expected to vest 5 years 6 months 6 years 6 months
Weighted average exercise price, options fully vested and expected to vest (in dollars per share) $ 19.26 $ 19.58
Aggregate intrinsic value, options fully vested and expected to vest $ 55,106 $ 37,581
Number of shares options, options exercisable 1,999,278 2,477,474
Weighted average remaining contractual life (in years), options exercisable 5 years 3 months 18 days 5 years 10 months 24 days
Weighted average exercise price, options exercisable (in dollars per share) $ 19.16 $ 19.24
Aggregate intrinsic value, options exercisable $ 50,257 $ 26,659