Workers Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Occupational disease
Accrued Workers Compensation [Line Items]
Service cost	$ 1,669	$ 1,860
Interest cost	1,354	1,195
Net benefit credit	3,023	3,055
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments	2,144	3,011
Total workers’ compensation expense
Accrued Workers Compensation [Line Items]
Total workers’ compensation expense	$ 5,167	$ 6,066