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Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Supplemental Executive Retirement Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 35 $ 33 $ 107 $ 100
Interest cost 79 71 237 211
Amortization of prior service cost 47 46 141 140
Amortization of losses 5 2 14 5
Net periodic cost 166 152 499 456
Directors Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 11 12 31 35
Interest cost 16 13 49 40
Amortization of prior service cost 10 18 29 54
Amortization of losses 7 15 21 45
Net periodic cost $ 44 $ 58 $ 130 $ 174