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Summary of Stock Option (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Options      
Beginning balance 2,326,627 2,371,650 2,528,177
Grants 89,600 394,400 390,800
Exercises (110,375) (365,223) (247,827)
Cancellations (37,750) (32,200) (68,000)
Expirations (25,900) (42,000) (231,500)
Ending balance 2,242,202 2,326,627 2,371,650
Options exercisable at End of Period 1,725,944    
Weighted-average exercise price      
Beginning balance $ 14.19 $ 12.75 $ 13.06
Grants 13.99 18.83 12.26
Exercises 8.84 8.63 4.91
Cancellations 15.57 12.23 16.89
Expirations 26.60 26.61 22.46
Ending balance 14.28 $ 14.19 $ 12.75
Options exercisable at End of Period $ 13.85    
Weighted-average remaining contractual life (years)      
Options outstanding, Ending balance 5 years 1 month 6 days    
Options exercisable, Ending balance 4 years 3 months 18 days    
Aggregate intrinsic value      
Options outstanding, end of period $ 3,715,000    
Options exercisable, end of period $ 3,551,000