Workers Compensation Expense (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 29, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accrued Workers Compensation [Line Items]
Additional collateral amount	$ 71,100
Number of days required to submit additional collateral to retain self-insurance status	30 days
Occupational disease
Accrued Workers Compensation [Line Items]
Service cost		$ 1,949	$ 1,892	$ 5,847	$ 5,674
Interest cost		1,110	1,398	3,330	4,196
Amortization of other actuarial losses (gains)		590	297	1,772	892
Net benefit cost (credit)		3,649	3,587	10,949	10,762
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments		1,779	2,030	5,449	6,312
Total workers' compensation expense
Accrued Workers Compensation [Line Items]
Total workers' compensation expense		$ 5,428	$ 5,617	$ 16,398	$ 17,074