Workers Compensation Expense (Workers Compensation Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Occupational disease
Accrued Workers Compensation [Line Items]
Service cost	$ 1,860	$ 1,559	$ 3,720	$ 3,117
Interest cost	1,194	1,168	2,389	2,337
Net benefit credit	3,054	2,727	6,109	5,454
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments	2,188	2,532	5,199	5,410
Total workers’ compensation expense
Accrued Workers Compensation [Line Items]
Total workers’ compensation expense	$ 5,242	$ 5,259	$ 11,308	$ 10,864