Workers Compensation Expense (Workers Compensation Expense) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Occupational disease
Accrued Workers Compensation [Line Items]
Service cost	$ 1,860	$ 1,558
Interest cost	1,195	1,169
Net benefit credit	3,055	2,727
Traumatic injury claims and assessments
Accrued Workers Compensation [Line Items]
Traumatic injury claims and assessments	3,011	2,878
Workers compensation
Accrued Workers Compensation [Line Items]
Total workers’ compensation expense	$ 6,066	$ 5,605