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Summary of Stock Option (Detail) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Options      
Beginning Balance 2,475,750 2,219,200 1,786,667
Grants 305,000 391,500 573,000
Exercises (199,823) (123,500) (39,250)
Cancellations (52,750) (11,450) (101,217)
Ending Balance 2,528,177 2,475,750 2,219,200
Options exercisable at End of Period 1,616,052    
Weighted-average exercise price      
Beginning Balance $ 12.62 $ 12.46 $ 12.14
Grants $ 11.64 $ 11.20 $ 13.12
Exercises $ 5.47 $ 5.19 $ 4.44
Cancellations $ 12.82 $ 13.29 $ 13.65
Ending Balance $ 13.06 $ 12.62 $ 12.46
Options exercisable at End of Period $ 14.19    
Weighted Average Remaining Contractual Term      
Options outstanding, Ending Balance 6 years 1 month 17 days    
Options exercisable, Ending balance 4 years 11 months 16 days    
Aggregate Intrinsic Value      
Options outstanding, end of period $ 4,525,905    
Options exercisable, end of period $ 3,813,485