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Postretirement Benefit Plans (Net Funded Status) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Postretirement Health And Life Coverage [Member]      
Change in benefit obligations:      
Benefit obligations $ 540 $ 572  
Change in benefit obligations: Service cost 4 4 $ 13
Change in benefit obligations: Interest cost 20 23 32
Change in benefit obligations: Change in experience (11) 2  
Change in benefit obligations: Change in assumptions 20 13  
Change in benefit obligations/fair value of plan assets: Benefit payments (65) (74)  
Benefit obligations 508 540 572
Change in fair value of plan assets:      
Change in fair value of plan assets: Employer contributions 65 74  
Change in benefit obligations/fair value of plan assets: Benefit payments (65) (74)  
Funded status at year end (508) (540)  
Director's Retirement Plan [Member]      
Change in benefit obligations:      
Benefit obligations 1,122 1,150  
Change in benefit obligations: Service cost 35 34 33
Change in benefit obligations: Interest cost 43 46 45
Change in benefit obligations: Actuarial gain (loss) 6 (13)  
Change in benefit obligations: Change in assumptions 2 (4)  
Change in benefit obligations/fair value of plan assets: Benefit payments (92) (91)  
Benefit obligations 1,116 1,122 $ 1,150
Change in fair value of plan assets:      
Change in fair value of plan assets: Employer contributions 92 91  
Change in benefit obligations/fair value of plan assets: Benefit payments (92) (91)  
Funded status at year end $ (1,116) $ (1,122)