Workers Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Occupational disease
Accrued Workers Compensation [Line Items]
Service cost	$ 1,669	$ 1,860	$ 3,338	$ 3,720
Interest cost	1,354	1,194	2,708	2,389
Net benefit credit	3,023	3,054	6,046	6,109
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments	2,410	2,188	4,554	5,199
Total workers’ compensation expense
Accrued Workers Compensation [Line Items]
Total workers’ compensation expense	$ 5,433	$ 5,242	$ 10,600	$ 11,308