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Benefit Plans - Components of Accumulated Other Comprehensive Loss (Gain) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Pension [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Unrecognized prior service cost $ 0 $ 0
Unrecognized net actuarial loss (gain) 11,091 11,968
Total accumulated other comprehensive loss (gain) 11,091 11,968
Post-retirement [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Unrecognized prior service cost 0 0
Unrecognized net actuarial loss (gain) (4,781) (6,993)
Total accumulated other comprehensive loss (gain) $ (4,781) $ (6,993)