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Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Supplemental Executive Retirement Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 33 $ 26 $ 100 $ 79
Interest cost 71 73 211 218
Amortization of losses 2   5  
Amortization of prior service cost 46 47 140 140
Net periodic cost 152 146 456 437
Directors Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 12 6 35 17
Interest cost 13 12 40 35
Amortization of losses 15   45  
Amortization of prior service cost 18 18 54 54
Net periodic cost $ 58 $ 36 $ 174 $ 106