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Net Periodic Benefit Cost (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Supplemental Executive Retirement Plan
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 34 $ 38 $ 68 $ 77
Interest cost 62 50 123 101
Amortization of losses 12 11 24 22
Amortization of prior service cost 47 33 94 67
Net periodic cost 155 132 309 267
Directors Plan
        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 6 6 13 13
Interest cost 11 10 21 20
Amortization of prior service cost 23 23 45 45
Net periodic cost $ 40 $ 39 $ 79 $ 78