Retirement Plans - Accumulated Other Comprehensive Loss (Details) - Postretirement Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans, Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Accumulated other comprehensive income (loss) before tax, beginning of period	$ (101)	$ (147)	$ (193)
Amortization of prior service credit (recognized in net periodic benefit cost)	46	46	46
Plan amendment	(449)	0	0
Accumulated other comprehensive income (loss) before tax, end of period	$ (504)	$ (101)	$ (147)