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Stock Options and Other Equity-Based Incentive Instruments - Summary of Options Outstanding (Detail) - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of options 428,900 446,400
Weighted average exercise price $ 16.85 $ 16.84
Intrinsic value (in thousands) $ 8,837  
Weighted average remaining contractual term (yrs.) 3 years 6 months  
Currently Exercisable [Member]    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of options 425,900  
Weighted average exercise price $ 16.80  
Intrinsic value (in thousands) $ 8,794  
Weighted average remaining contractual term (yrs.) 3 years 6 months  
Currently Not Exercisable [Member]    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of options 3,000  
Weighted average exercise price $ 23.21  
Intrinsic value (in thousands) $ 43  
Weighted average remaining contractual term (yrs.) 6 years 3 months 18 days