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Stock Option Activity (Detail) (USD $)
In Thousands, except Share data, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Option shares      
Options Outstanding at beginning of period 479,443    
Granted 0   0
Exercised 5,223 2,969 2,631
Forfeited 2,340    
Options Outstanding at end of period 471,880 479,443  
Options Exercisable at end of period 391,013    
Weighted average exercise price per share      
Options Outstanding at beginning of period $ 20.78    
Granted       
Exercised $ 11.97    
Forfeited $ 21.85    
Options Outstanding at end of period $ 20.87 $ 20.78  
Options Exercisable at end of period $ 22.69    
Weighted average remaining contractual term (years)      
Options Outstanding at end of period 6 years 2 months 12 days    
Options Exercisable at end of period 5 years 8 months 12 days    
Aggregate Intrinsic Value      
Options Outstanding at end of period $ 432    
Options Exercisable at end of period $ 147