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Pension and Other Postretirement Benefit Plans (Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates) (Details) - Other Postretirement Benefits
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Effect on total of service and interest cost components one percentage point increase $ 3,536
Effect on total of service and interest cost components one percentage point decrease (2,962)
Effect on accumulated postretirement benefit obligation one percentage point increase 71,922
Effect on accumulated postretirement benefit obligation one percentage point decrease $ (60,924)