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Summary of Stock Option Activity (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Number of options and SAR units    
Number of shares outstanding, Beginning balance [1] 729,017  
Number of shares, Options/SAR units granted [1] 0  
Number of shares, Options/SAR units exercised [1] (19,474)  
Number of shares, Options/SAR units forfeited or expired [1] (2,128)  
Number of shares outstanding, Ending balance [1] 707,415 [2] 729,017
Number of shares exercisable, Ending balance [1],[3] 610,731  
Weighted-average exercise price    
Weighted-average exercise price, Beginning balance $ 19.77  
Weighted average exercise price, Granted 0  
Weighted average exercise price, Exercised 16.22  
Weighted average exercise price, Forfeited or expired 20.50  
Weighted average exercise price, Ending balance 19.87 [2] $ 19.77
Weighted average exercise price, exercisable [3] $ 18.96  
Weighted average remaining contractual term    
Weighted average remaining contractual life, Outstanding at beginning of period 4 years 6 months [2] 5 years 2 months 12 days
Weighted average remaining contractual life, Exercisable at end of period [3] 4 years 2 months 12 days  
Aggregate intrinsic value    
Aggregate intrinsic value, Outstanding, at beginning of period $ 6,447 [2] $ 19,229
Aggregate intrinsic value, Exercisable at ending of period [3] $ 6,089  
[1] The SAR units are convertible for a maximum number of shares of the Company's common stock equal to 75% of the SAR units subject to the grant.
[2] Due to the ceiling imposed on the SAR grants, the outstanding amount equals a maximum of 645,874 shares of the Company's common stock issuable upon exercise.
[3] Due to the ceiling imposed on the SAR grants, the exercisable amount equals a maximum of 553,861 shares of the Company's common stock issuable upon exercise.