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Stock Options and Other Equity-Based Incentive Instruments - Summary of Options Outstanding (Detail) - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of options 458,400 592,250
Weighted average exercise price $ 16.85 $ 17.12
Intrinsic value (in thousands) $ 10,958  
Weighted average remaining contractual term (yrs.) 4 years 1 month 6 days  
Currently Exercisable [Member]    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of options 431,100  
Weighted average exercise price $ 16.61  
Intrinsic value (in thousands) $ 10,406  
Weighted average remaining contractual term (yrs.) 4 years  
Currently Not Exercisable [Member]    
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]    
Number of options 27,300  
Weighted average exercise price $ 20.54  
Intrinsic value (in thousands) $ 552  
Weighted average remaining contractual term (yrs.) 6 years 1 month 6 days