Workers Compensation Expense (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Feb. 29, 2020	Mar. 31, 2021	Mar. 31, 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	$ 1,891
Interest cost		1,110	1,399
Amortization of other actuarial losses (gains)		591	297
Net benefit cost (credit)		3,650	3,587
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments		1,805	2,182
Total workers' compensation expense
Accrued Workers Compensation [Line Items]
Total workers' compensation expense		$ 5,455	$ 5,769