Retirement Plans - Other Postretirement Benefit Plan (Details) - Other Postretirement Benefits Plan [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure
Service cost	$ 0	$ 0	$ 1	$ 1
Interest cost	39	40	78	80
Amortization of gain	(11)	(25)	(23)	(50)
(Income) expense, net	$ 28	$ 15	$ 56	$ 31