Pension and Other Postretirement Benefits - Benefits Expected to be Paid in Subsequent Years from Our Pension and Other Postretirement as Well as Medicare Subsidy Receipts (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Pension Benefits [Member]
Pension And Other Employee Benefit Plans [Line Items]
2018	$ 16,562
2019	16,923
2020	17,937
2021	17,688
2022	15,787
2023-2027	83,382
Total	168,279
Other Benefits [Member]
Pension And Other Employee Benefit Plans [Line Items]
2018	1,580
2019	1,568
2020	1,562
2021	1,557
2022	1,553
2023-2027	7,753
Total	$ 15,573