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Summary of Stock Options Summarized by Exercise Price (Detail) - Incentive Stock Options - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Outstanding Weighted Average, Shares 472,783      
Outstanding Weighted Average, Remaining Life (Months) 57 months      
Outstanding Weighted Average, Exercise Price $ 9.38 $ 9.33 $ 9.25 $ 9.22
Exercisable Weighted Average, Shares 472,783      
Exercisable Weighted Average, Exercise Price $ 9.38 $ 7.97 $ 7.73 $ 7.82
Range One        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise Price Per Share, Lower Range $ 7.50      
Outstanding Weighted Average, Shares 249,050      
Outstanding Weighted Average, Remaining Life (Months) 35 months      
Outstanding Weighted Average, Exercise Price $ 7.50      
Exercisable Weighted Average, Shares 249,050      
Exercisable Weighted Average, Exercise Price $ 7.50      
Range Two        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Exercise Price Per Share, Lower Range 11.32      
Exercise Price Per Share, Upper Range $ 14.75      
Outstanding Weighted Average, Shares 223,733      
Outstanding Weighted Average, Remaining Life (Months) 82 months      
Outstanding Weighted Average, Exercise Price $ 11.48      
Exercisable Weighted Average, Shares 223,733      
Exercisable Weighted Average, Exercise Price $ 11.48