Retirement Plans, Amounts Recognized in Accumulated Other Comprehensive Income Table (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Retirement Account Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	$ (295)	$ (342)
Net actuarial loss	24,926	35,443
Amount not yet recognized in net periodic benefit cost, but recognized in accumulated other comprehensive loss	24,631	35,101
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service cost	(531)	(580)
Net actuarial loss	205	79
Amount not yet recognized in net periodic benefit cost, but recognized in accumulated other comprehensive loss	$ (326)	$ (501)