Retirement Plans - Net Periodic Pension Cost - Pension Plan (Details) - Pension plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cost components:
Service cost	$ (15)	$ (223)	$ (265)
Interest cost	(386)	(406)	(466)
Expected return on plan assets	531	517	615
Net amortization and deferral	(122)	(184)	(284)
Settlement recognition	47	0	(536)
Total net periodic pension cost	$ 55	$ (296)	$ (936)