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Employee Benefit Plans (Schedule of Effect of One-percentage Point Change in Assumed Health Care Cost Trend Rates) (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Pension Benefit Plan [Member]  
Effect on Current Year's Service and Interest Cost - Increase $ 649
Effect on Projected Benefit Obligation - Increase 9,412
Other Benefits Plan [Member]  
Effect on Current Year's Service and Interest Cost - Decrease (499)
Effect on Projected Benefit Obligation - Decrease $ (7,403)