Coal Workers' Pneumoconiosis and Workers' Compensation (Schedule of Components of Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 3,791	$ 6,650	$ 5,122
Interest cost	7,001	5,245	4,050
Recognized net actuarial loss (gain)	1,016	(853)	(7,631)
State administrative fees and insurance bond premiums	0	0	0
Net periodic benefit (credit) cost	11,808	11,042	1,541
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5,685	6,230	5,734
Interest cost	2,585	2,283	2,321
Recognized net actuarial loss (gain)	(774)	(79)	(598)
State administrative fees and insurance bond premiums	2,157	2,671	3,198
Net periodic benefit (credit) cost	$ 9,653	$ 11,105	$ 10,655