Postretirement Benefits - Pension Liability (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liabilities for defined benefit pension plan	$ 595,347	$ 602,606
Qualified plan [Member] | Funded plan [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liabilities for defined benefit pension plan	509,965	518,141
Nonqualified plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liabilities for defined benefit pension plan	85,382	84,465
Nonqualified plan [Member] | SERPs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liabilities for defined benefit pension plan	$ 85,382	$ 84,465