Postretirement Benefits - Pension Liability (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liabilities for defined benefit pension plan	$ 584,653	$ 556,199
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	509,998	481,792
Nonqualified plan [Member] | SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liabilities for defined benefit pension plan	$ 74,655	$ 74,407