Workers Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accrued Workers Compensation [Line Items]
Reduction to workers' compensation liability	$ 4,000		$ 4,000
Data discount rate	3.08%
Occupational disease
Accrued Workers Compensation [Line Items]
Service cost	$ 1,860	$ 1,558	5,580	$ 4,675
Interest cost	1,196	1,169	3,585	3,506
Curtailments	0	(4,660)	0	(4,660)
Net benefit credit	3,056	(1,933)	9,165	3,521
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments	(2,069)	3,077	3,130	8,487
Total workers’ compensation expense
Accrued Workers Compensation [Line Items]
Total workers’ compensation expense	$ 987	$ 1,144	$ 12,295	$ 12,008