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Post Retirement Benefits - Change in Benefit Obligation (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Change in benefit obligation:      
Interest cost $ 298,000 $ 323,000 $ 316,000
Postretirement benefits      
Change in benefit obligation:      
Benefit obligation at beginning of year 8,667,000 9,006,000  
Interest cost 298,000 323,000  
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) (417,000) (320,000)  
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax (2,869,000) (3,634,000)  
Defined Benefit Plan, Benefit Obligation, Benefits Paid 332,000 342,000  
Benefit obligation at beginning of year 9,050,000 8,667,000 $ 9,006,000
Plan Assets 0 0  
Amounts recorded in accumulated other comprehensive income:      
Prior service credit (6,602,000) (7,098,000)  
Net loss 3,733,000 3,464,000  
Total $ (2,869,000) $ (3,634,000)  
Weighted-average assumptions:      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 3.40% 3.80%  
Defined Benefit Plan, Health Care Cost Trend Rate Assumed, Next Fiscal Year 7.00% 7.00%  
Defined Benefit Plan, Ultimate Health Care Cost Trend Rate 5.00% 5.00%