Workers' Compensation Expense (Worker's Compensation Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accrued Workers' Compensation [Line Items]
Service cost	$ 6,677	$ 7,440	$ 6,320
Interest cost	4,922	4,365	4,651
Net amortization	0	0	0
Traumatic and other workers’ compensation claims
Accrued Workers' Compensation [Line Items]
Total workers’ compensation expense	24,649	17,200	14,179
Total occupational disease
Accrued Workers' Compensation [Line Items]
Service cost	6,677	7,440
Interest cost	4,922	4,365
Total occupational disease	11,599	11,805	10,971
Traumatic injury claims and assessments
Accrued Workers' Compensation [Line Items]
Traumatic injury claims and assessments	$ 13,050	$ 5,395	$ 3,208