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Savings Plans, Pension Plans and Other Postretirement Employee Benefits (One Percent Point Change in Health Care Cost Trend Rate Effect) (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Savings Plans Pension Plans And Other Postretirement Employee Benefits [Abstract]  
Effect on total service and interest cost components, 1% increase $ 28
Effect on total service and interest cost components, 1% decrease (24)
Effect on accumulated postretirement benefit obligation, 1% increase 443
Effect on accumulated postretirement benefit obligation, 1% decrease $ (373)