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Benefit Plans - Net Periodic Benefit Cost (Increase) (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 0 $ 0 $ 0
Interest cost 1,227 1,247 1,137
Return on plan assets (2,550) (2,449) (2,530)
Amortization of net gain (loss) 920 943 774
Amortization of unrecognized prior service cost 0 0 0
Net periodic benefit cost (increase) (403) (259) (619)
Post-retirement [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 105 150 168
Interest cost 871 1,138 1,122
Return on plan assets 0 0 0
Amortization of net gain (loss) (677) 0 1
Amortization of unrecognized prior service cost 0 0 (1)
Net periodic benefit cost (increase) $ 299 $ 1,288 $ 1,290