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Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Supplemental Executive Retirement Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 36 $ 34 $ 72 $ 67
Interest cost 79 70 158 140
Amortization of prior service cost 47 47 94 94
Amortization of losses 4 1 9 3
Net periodic cost 166 152 333 304
Directors Plan        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 9 11 20 23
Interest cost 16 14 33 27
Amortization of prior service cost 9 18 19 36
Amortization of losses 5 15 14 30
Net periodic cost $ 39 $ 58 $ 86 $ 116