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Employee Benefit Plans - Accumulated Post-Retirement Benefit Obligation, Funded Status, and Net Periodic Benefit Cost (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Change in Benefit Obligations [Roll Forward]            
Benefit obligation at beginning of year $ 1,870 $ 1,967 $ 1,928      
Interest cost 77 81 80      
Benefits paid (113) (109) (102)      
Actuarial (gain) loss       $ 40 $ (69) $ 61
Benefit obligation at end of period 1,874 1,870 1,967      
Funded Status of Plan [Abstract]            
Benefit obligation at end of period (1,870) (1,967) (1,928) (1,874) (1,870) (1,967)
Unamortized loss       186 156 240
Accrued benefit cost       $ (1,688) $ (1,714) $ (1,727)
Weighted average discount rate as of December 31       0.0425 0.0425 0.0425
Components of Net Periodic Benefit Cost [Abstract]            
Interest cost 77 81 80      
Amortization of loss 0 4 0      
Other settlement expense 11 11 12      
Net periodic benefit cost $ 88 $ 96 $ 92      
Weighted average discount rate for net periodic cost 4.25% 4.25% 4.25%