Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Pension plan
Cost components:
Service cost-benefits earned during the period	$ 0	$ (45)
Interest cost on projected benefit obligation	(182)	(198)
Expected return on plan assets	292	263
Net amortization and deferral	(32)	(73)
Total net periodic pension earnings (cost)	78	(53)
SERP
Cost components:
Interest cost on projected benefit obligation	(128)	(146)
Net amortization and deferral	(32)	(166)
Total net periodic pension earnings (cost)	$ (160)	$ (312)