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Pension and Other Postretirement Benefits - Effect of One Percentage - Point Change in Assumed Health Care Cost Trend Rates (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Postemployment Benefits [Abstract]  
Effect on total of service and interest cost components, 1% Increase $ 127
Effect on postretirement benefit obligation, 1% Increase 2,306
Effect on total of service and interest cost components, 1% Decrease (103)
Effect on postretirement benefit obligation, 1% Decrease $ (1,943)