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Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Supplemental Executive Retirement Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 133 $ 106 $ 135
Interest cost 281 290 246
Amortization of losses 7   49
Amortization of prior service cost 187 187 187
Net periodic cost 608 583 617
Directors Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 47 22 26
Interest cost 53 46 41
Amortization of losses 60   1
Amortization of prior service cost 72 73 90
Net periodic cost $ 232 $ 141 $ 158