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Pension, Health Care and Postretirement Benefits Other Than Pensions - Effects of Trend Rate Changes and Future Benefit Cash Payments (Details) - Postretirement Benefits Other than Pensions
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Significant effect on amounts reported for service and interest rate component and postretirement health care benefit obligation  
Effect on total of service and interest cost components, one-percentage-point, increase $ 104
Effect on total of service and interest cost components, one-percentage-point, decrease (124)
Effect on the postretirement benefit obligation one-percentage-point, increase 2,655
Effect on the postretirement benefit obligation one-percentage-point, decrease (2,794)
Retiree health care benefit cash payments  
2018 17,192
2019 18,139
2020 19,058
2021 19,541
2022 19,816
2023 through 2027 98,270
Total expected benefit cash payments $ 192,016