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Benefit Plans Benefits Expected to be Paid in Next Five Years (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Pension Plan [Member]  
Expected Future Benefit Payments [Abstract]  
2017 $ 6,941
2018 7,032
2019 7,392
2020 7,741
2021 8,013
2022 - 2026 43,828
Other Postretirement Benefits Plan [Member]  
Expected Future Benefit Payments [Abstract]  
2017 795
2018 817
2019 823
2020 868
2021 916
2022 - 2026 5,032
Maximum [Member]  
Defined Benefit Plan Disclosure [Line Items]  
Estimated required and discretionary employer cash contributions in 2017 $ 7,593