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Summary of Assets Reported at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale $ 422,196 $ 406,471
Fair Value, Measurements, Recurring
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 422,196 406,471
Fair Value, Measurements, Recurring | US Government agencies
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 1,288 2,026
Fair Value, Measurements, Recurring | Obligations of states and political subdivisions
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 80,769 71,340
Fair Value, Measurements, Recurring | U.S. Government agencies collateralized by mortgage obligations
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 338,330 331,758
Fair Value, Measurements, Recurring | Other Securities
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 1,809 1,347
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 1,809 1,347
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Other Securities
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 1,809 1,347
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 420,387 405,124
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | US Government agencies
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 1,288 2,026
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Obligations of states and political subdivisions
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 80,769 71,340
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | U.S. Government agencies collateralized by mortgage obligations
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale 338,330 331,758
Fair Value, Measurements, Recurring | Fair Value, Gain Loss
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale   (1,370)
Fair Value, Measurements, Recurring | Fair Value, Gain Loss | Other Securities
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Investment securities available for sale   $ (1,370)