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Summary of Stock Option (Detail) (USD $)
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Options      
Beginning balance 2,528,177 2,475,750 2,219,200
Grants 390,800 305,000 391,500
Exercises (247,827) (199,823) (123,500)
Cancellations (68,000) (52,750) (11,450)
Expirations (231,500)    
Ending balance 2,371,650 2,528,177 2,475,750
Options exercisable at End of Period 1,508,350    
Weighted-average exercise price      
Beginning balance $ 13.06 $ 12.62 $ 12.46
Grants $ 12.26 $ 11.64 $ 11.20
Exercises $ 4.91 $ 5.47 $ 5.19
Cancellations $ 16.89 $ 12.82 $ 13.29
Expirations $ 22.46    
Ending balance $ 12.75 $ 13.06 $ 12.62
Options exercisable at End of Period $ 13.15    
Weighted-average remaining contractual life (years)      
Options outstanding, Ending balance 6 years 2 months 16 days    
Options exercisable, Ending balance 5 years 3 months 22 days    
Aggregate intrinsic value      
Options outstanding, end of period $ 10,968,922    
Options exercisable, end of period $ 7,800,990