Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension plan
Earnings (cost) components:
Interest cost	$ (97)	$ (91)
Expected return on plan assets	131	146
Net amortization and deferral	(19)	(16)
Total net periodic pension earnings	15	39
SERP
Earnings (cost) components:
Interest cost	(76)	(64)
Net amortization and deferral	(16)	(16)
Total net periodic pension earnings	$ (92)	$ (80)