Retirement Plans - Net Periodic Pension Cost - Pension Plan (Details) - Pension plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cost components:
Service cost	$ 0	$ 0	$ 0
Interest cost	(338)	(389)	(365)
Expected return on plan assets	643	522	585
Net amortization and deferral	(68)	(75)	(64)
Settlement recognition	0	0	0
Total net periodic pension earnings/(cost)	$ 237	$ 58	$ 156