Workers' Compensation Expense (Worker's Compensation Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accrued Workers' Compensation [Line Items]
Service cost	$ 7,564	$ 6,677	$ 7,440
Interest cost	5,115	4,922	4,365
Net amortization	1,189	0	0
Total workers' compensation expense
Accrued Workers' Compensation [Line Items]
Total workers' compensation expense	26,790	24,649	17,200
Occupational disease
Accrued Workers' Compensation [Line Items]
Service cost	7,564	6,677
Interest cost	5,115	4,922
Net benefit cost (credit)	13,868	11,599	11,805
Traumatic injury claims and assessments
Accrued Workers' Compensation [Line Items]
Traumatic injury claims and assessments	$ 12,922	$ 13,050	$ 5,395