Workers Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Feb. 29, 2020
Accrued Workers Compensation [Line Items]
Additional collateral amount			$ 71,100
Occupational disease
Accrued Workers Compensation [Line Items]
Service cost	$ 1,891	$ 1,669
Interest cost	1,399	1,354
Net amortization(1)	297	0
Net benefit credit	3,587	3,023
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments	2,182	2,144
Total workers' compensation expense
Accrued Workers Compensation [Line Items]
Total workers' compensation expense	$ 5,769	$ 5,167