Workers Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Occupational disease
Accrued Workers Compensation [Line Items]
Service cost	$ 1,670	$ 1,860	$ 5,008	$ 5,580
Interest cost	1,354	1,196	4,062	3,585
Net benefit credit	3,024	3,056	9,070	9,165
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments	2,088	(2,069)	6,642	3,130
Total workers’ compensation expense
Accrued Workers Compensation [Line Items]
Total workers’ compensation expense	$ 5,112	$ 987	$ 15,712	$ 12,295