Workers' Compensation Expense (Schedule of Amounts Recognized in Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accrued Workers' Compensation [Line Items]
Total obligations	$ 259,954	$ 231,718
Less amount included in accrued expenses	15,259	16,119
Noncurrent obligations	244,695	215,599
Occupational disease
Accrued Workers' Compensation [Line Items]
Occupational disease costs	183,001	158,325	$ 118,900
Total workers' compensation expense
Accrued Workers' Compensation [Line Items]
Occupational disease costs	$ 76,953	$ 73,393