Workers Compensation Expense (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 29, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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		$ 1,891	$ 1,669	$ 3,782	$ 3,338
Interest cost		1,399	1,354	2,798	2,708
Net amortization(1)		298	0	595	0
Net benefit credit		3,588	3,023	7,175	6,046
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments		2,100	2,410	4,282	4,554
Total workers' compensation expense
Accrued Workers Compensation [Line Items]
Total workers' compensation expense		$ 5,688	$ 5,433	$ 11,457	$ 10,600