Workers Compensation Expense - Worker's compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Total workers' compensation expense
Accrued Workers' Compensation [Line Items]
Total workers' compensation expense	$ 5,004	$ 4,956	$ 8,956	$ 9,819
Occupational disease
Accrued Workers' Compensation [Line Items]
Service cost	994	1,497	1,987	2,995
Interest cost	1,510	1,152	3,020	2,304
Net amortization	241	(157)	482	(314)
Net benefit credit	2,263	2,806	4,525	5,613
Traumatic injury claims and assessments
Accrued Workers' Compensation [Line Items]
Traumatic injury claims and assessments	$ 2,741	$ 2,150	$ 4,431	$ 4,206