Coal Workers' Pneumoconiosis and Workers' Compensation (Schedule of Components of Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CWP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 5,122	$ 4,327	$ 6,194
Interest cost	4,050	4,283	5,116
Recognized net actuarial loss	(7,631)	(4,948)	(5,576)
State administrative fees and insurance bond premiums	0	0	0
Curtailment gain	0	(1,307)	0
Net periodic benefit cost (credit)	1,541	2,355	5,734
Workers' Compensation
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5,734	7,466	9,201
Interest cost	2,321	2,499	3,131
Recognized net actuarial loss	(598)	(395)	(30)
State administrative fees and insurance bond premiums	3,198	3,199	3,510
Curtailment gain	0	0	0
Net periodic benefit cost (credit)	$ 10,655	$ 12,769	$ 15,812