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Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Supplemental Executive Retirement Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 34 $ 27 $ 67 $ 53
Interest cost 70 72 140 145
Amortization of losses 1   3  
Amortization of prior service cost 47 46 94 93
Net periodic cost 152 145 304 291
Directors Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 11 6 23 11
Interest cost 14 11 27 23
Amortization of losses 15   30  
Amortization of prior service cost 18 18 36 36
Net periodic cost $ 58 $ 35 $ 116 $ 70