Workers' Compensation Expense (Occupational Disease Costs Activity and Assumptions) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Self Insurance Reserve [Roll Forward]
Service cost	$ 7,564	$ 6,677	$ 7,440
Interest cost	$ 5,115	$ 4,922	4,365
Discount Rate
Self Insurance Reserve [Roll Forward]
Discount rate	0.0248	0.0331
Occupational disease
Self Insurance Reserve [Roll Forward]
Beginning of period	$ 158,325	$ 118,900
Service cost	7,564	6,677
Interest cost	5,115	4,922
Actuarial (gain) loss	19,327	35,993
Benefit and administrative payments	(7,330)	(8,167)
End of period	$ 183,001	$ 158,325	$ 118,900