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Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Supplemental Executive Retirement Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 36 $ 33
Interest cost 79 70
Amortization of prior service cost 47 47
Amortization of losses 5 2
Net periodic cost 167 152
Directors Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 11 12
Interest cost 17 13
Amortization of prior service cost 10 18
Amortization of losses 9 15
Net periodic cost $ 47 $ 58