Retirement Plans - Net Periodic Pension Cost - SERP (Details) - SERP liability adjustments - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ (229)	$ (303)	$ (268)
Net amortization and other	(86)	(62)	(223)
Total net periodic pension earnings/(cost)	$ (315)	$ (365)	$ (491)