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Postretirement Benefit Plans (Amounts Recognized in Other Comprehensive (Loss) Income) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Postretirement Health And Life Coverage [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Net loss (gain), pretax $ (31) $ (47)
Net prior service cost, pretax (209) (244)
Director's Retirement Plan [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Net loss (gain), pretax 60 77
Net prior service cost, pretax $ 43 $ 64