Workers Compensation Expense (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 29, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
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		$ 1,892	$ 1,670	$ 5,674	$ 5,008
Interest cost		1,398	1,354	4,196	4,062
Net amortization(1)		297	0	892	0
Net benefit credit		3,587	3,024	10,762	9,070
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments		2,030	2,088	6,312	6,642
Total workers' compensation expense
Accrued Workers Compensation [Line Items]
Total workers' compensation expense		$ 5,617	$ 5,112	$ 17,074	$ 15,712