Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Pension plan
Cost components:
Service cost-benefits earned during the period	$ 0	$ (30)
Interest cost on projected benefit obligation	(273)	(292)
Expected return on plan assets	438	397
Net amortization and deferral	(48)	(74)
Total net periodic pension earnings (cost)	117	1
SERP
Cost components:
Interest cost on projected benefit obligation	(192)	(216)
Net amortization and deferral	(48)	(189)
Total net periodic pension earnings (cost)	$ (240)	$ (405)