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Net Periodic Benefit Cost (Detail) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Supplemental Executive Retirement Plan
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 153 $ 161  
Interest cost 203 224  
Amortization of losses (gains) 45    
Amortization of prior service cost 134 134  
Net periodic cost 535 519 424
Directors Plan
      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 27 29  
Interest cost 39 43  
Amortization of prior service cost 90 90  
Net periodic cost $ 156 $ 162 $ 259