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Other Postretirement Benefits - Schedule of Effect of One Percentage-Point Change in Assumed Health Care Cost Trend Rates (Details) - Other Postretirement Benefit Plans [Member]
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rate [Abstract]  
Effect on total service and interest cost (increase) $ 62
Effect on accumulated postretirement benefit obligation ( increase) 2,200
Effect on total service and interest cost (decrease) (53)
Effect on accumulated postretirement benefit obligation (decrease) $ (1,865)