Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Pension plan
Earnings (cost) components:
Interest cost	$ (255)	$ (291)
Expected return on plan assets	483	393
Net amortization and deferral	(51)	(57)
Total net periodic pension earnings	177	45
SERP
Earnings (cost) components:
Interest cost	(171)	(228)
Net amortization and deferral	(66)	(48)
Total net periodic pension earnings	$ (237)	$ (276)