Postretirement Benefit Plans - Pension Liability (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liabilities for defined benefit pension plan	$ 594,354	$ 602,605
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	509,526	518,140
Nonqualified plan [Member] | SERPs [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liabilities for defined benefit pension plan	$ 84,828	$ 84,465