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Benefit Plans - Benefit Obligation and Plan Asset Rollforward (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension [Member]      
Change in benefit obligation:      
Benefit obligation at beginning of year $ 29,533 $ 28,274 $ 28,921
Service cost 0 0 0
Interest cost 1,227 1,247 1,137
Actuarial loss 0 70 78
Benefits paid (1,590) (1,247) (1,179)
Change in actuarial assumptions 2,800 1,189 (683)
Benefit obligation at end of year 31,970 29,533 28,274
Change in plan assets:      
Fair value of plan assets at beginning of year 43,153 41,448 42,744
Actual return on plan assets 5,307 2,952 (117)
Employer contributions 0 0 0
Benefits paid (1,590) (1,247) (1,179)
Fair value of plan assets at end of year 46,870 43,153 41,448
Funded status at end of year 14,900 13,620 13,174
Post-retirement [Member]      
Change in benefit obligation:      
Benefit obligation at beginning of year 20,805 25,694 28,333
Service cost 105 150 168
Interest cost 871 1,138 1,122
Actuarial loss 0 0 122
Benefits paid (560) (682) (644)
Change in actuarial assumptions 1,536 (5,495) (3,407)
Benefit obligation at end of year 22,757 20,805 25,694
Change in plan assets:      
Fair value of plan assets at beginning of year 0 0 0
Actual return on plan assets 0 0 0
Employer contributions 560 682 644
Benefits paid (560) (682) (644)
Fair value of plan assets at end of year 0 0 0
Funded status at end of year $ (22,757) $ (20,805) $ (25,694)