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Additional Information with Respect to Options Granted, Vested and Exercised (Detail) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Weighted-average grant date fair value of stock options granted (per share)   $ 4.90 $ 5.79
Aggregate grant date fair value of stock options vested during the year $ 4,565 $ 4,729 $ 5,077
Aggregate intrinsic value of stock options exercised $ 209 $ 366