Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Pension plan
Earnings (cost) components:
Interest cost	$ (194)	$ (182)
Expected return on plan assets	262	292
Net amortization and deferral	(38)	(32)
Total net periodic pension earnings	30	78
SERP
Earnings (cost) components:
Interest cost	(152)	(128)
Net amortization and deferral	(32)	(32)
Total net periodic pension earnings	$ (184)	$ (160)