Postretirement Benefit Plans - Pension Liability (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Benefit Obligation	$ 548,579	$ 527,522
SERP (Closed) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Benefit Obligation	2,845	3,222
Funded plan [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Benefit Obligation	469,357	449,613
Nonqualified plan [Member] | SERP (Active) [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pension Benefit Obligation	$ 76,377	$ 74,687