Workers' Compensation Expense (Occupational Disease Costs Activity and Assumptions) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Self Insurance Reserve [Roll Forward]
Service cost	$ 6,677	$ 7,440	$ 6,320
Interest cost	$ 4,922	$ 4,365	4,651
Discount Rate
Self Insurance Reserve [Roll Forward]
Discount rate	0.0331	0.0426
Occupational disease costs
Self Insurance Reserve [Roll Forward]
Beginning of period	$ 118,900	$ 122,426
Service cost	6,677	7,440
Interest cost	4,922	4,365
Actuarial (gain) loss	35,993	(7,071)
Benefit and administrative payments	(8,167)	(8,260)
End of period	$ 158,325	$ 118,900	$ 122,426