Retirement Plans - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension plan
Earnings (cost) components:
Interest cost	$ (85)	$ (97)
Expected return on plan assets	161	131
Net amortization and deferral	(17)	(19)
Total net periodic pension earnings	59	15
SERP
Earnings (cost) components:
Interest cost	(57)	(76)
Net amortization and deferral	(22)	(16)
Total net periodic pension earnings	$ (79)	$ (92)