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Post-retirement Plans (Effect of One Percent Change in Health Care Cost Trend Rates) (Details) - Postretirement Health Coverage [Member]
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
One Percent Increase, Service Cost $ 175
One Percentage Point Decrease on Service Cost (146)
One Percentage Point Increase on Accumulated Post-retirement Benefit Obligation 1,393
One Percentage Point Decrease on Accumulated Post-retirement Benefit Obligation (1,204)
Interest Expense [Member]  
Defined Benefit Plan Disclosure [Line Items]  
One Percent Increase, Service Cost 48
One Percentage Point Decrease on Service Cost $ (42)