Coal Workers' Pneumoconiosis and Workers' Compensation (Schedule of Components of Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 6,650	$ 5,122	$ 4,327
Interest cost	5,245	4,050	4,283
Recognized net actuarial loss	(853)	(7,631)	(4,948)
State administrative fees and insurance bond premiums	0	0	0
Curtailment gain	0	0	(1,307)
Net periodic benefit (credit) cost	11,042	1,541	2,355
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	6,230	5,734	7,466
Interest cost	2,283	2,321	2,499
Recognized net actuarial loss	(79)	(598)	(395)
State administrative fees and insurance bond premiums	2,671	3,198	3,199
Curtailment gain	0	0	0
Net periodic benefit (credit) cost	$ 11,105	$ 10,655	$ 12,769