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Summary of Stock Option (Detail) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Options      
Beginning balance 1,456,200 1,775,400 2,242,202
Grants 205,750 125,750 66,850
Exercises (58,000) (300,000) (272,325)
Cancellations (22,375) (45,700) (30,750)
Expirations (32,750) (99,250) (230,577)
Ending balance 1,548,825 1,456,200 1,775,400
Options exercisable at End of Period 1,279,825    
Weighted-average exercise price      
Beginning balance $ 13.64 $ 13.44 $ 14.28
Grants 13.56 17.38 15.44
Exercises 4.93 11.34 9.01
Cancellations 16.95 16.40 15.39
Expirations 15.50 20.40 27.16
Ending balance 13.87 $ 13.64 $ 13.44
Options exercisable at End of Period $ 13.76    
Weighted-average remaining contractual life (years)      
Options outstanding, Ending balance 4 years 2 months 12 days    
Options exercisable, Ending balance 3 years 2 months 12 days    
Aggregate intrinsic value      
Options outstanding, end of period $ 450,500    
Options exercisable, end of period $ 450,500