Postretirement Benefit Plans - Pension Liability (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Benefit Obligation	$ 555,977	$ 527,522
Qualified plan [Member] | Funded plan [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Benefit Obligation	475,927	449,613
Nonqualified plan [Member] | SERP (Active) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Benefit Obligation	77,223	74,687
Nonqualified plan [Member] | SERP (Closed) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Benefit Obligation	$ 2,827	$ 3,222