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Net Periodic Benefit Cost (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Supplemental Executive Retirement Plan
    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 34 $ 39
Interest cost 61 51
Amortization of losses 12 11
Amortization of prior service cost 47 34
Net periodic cost 154 135
Directors Plan
    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 7 7
Interest cost 10 10
Amortization of prior service cost 22 22
Net periodic cost $ 39 $ 39