Workers' Compensation Expense (Occupational Disease Costs Activity and Assumptions) (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Self Insurance Reserve [Roll Forward]
Service cost	$ 1,583	$ 7,440	$ 6,320
Interest cost	1,126	$ 4,365	$ 4,651
Discount Rate
Self Insurance Reserve [Roll Forward]
Discount rate		0.0426	0.0366
Occupational disease costs
Self Insurance Reserve [Roll Forward]
Beginning of period		$ 122,426	$ 111,159
Service cost		7,440	6,320
Interest cost		4,365	4,651
Curtailments		0	(5,433)
Actuarial (gain) loss		(7,071)	12,242
Benefit and administrative payments		(8,260)	(6,513)
End of period	$ 111,159	$ 118,900	$ 122,426