Coal Workers' Pneumoconiosis and Workers' Compensation (Schedule of Changes in Benefit Obligation, Plan Assets and Funded Status) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CWP
Change in benefit obligation:
Benefit obligation at beginning of period	$ 118,836	$ 121,285
State administrative fees and insurance bond premiums	0	0	$ 0
Service cost	5,122	4,327	6,194
Interest cost	4,050	4,283	5,116
Actuarial loss	47,939	439
Benefits paid	(13,107)	(10,191)
Curtailment gain	0	(1,307)	0
Benefit obligation at end of period	162,840	118,836	121,285
Current assets	0	0
Current liabilities	(12,972)	(10,763)
Noncurrent liabilities	(149,868)	(108,073)
Net obligation recognized	(162,840)	(118,836)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial gain	(7,144)	(62,714)
Net amount recognized (before tax effect)	(7,144)	(62,714)
Workers' Compensation
Change in benefit obligation:
Benefit obligation at beginning of period	78,099	81,502
State administrative fees and insurance bond premiums	3,198	3,199	3,510
Service cost	5,734	7,466	9,201
Interest cost	2,321	2,499	3,131
Actuarial loss	3,553	121
Benefits paid	(14,377)	(16,688)
Curtailment gain	0	0	0
Benefit obligation at end of period	78,528	78,099	$ 81,502
Current assets	1,437	1,429
Current liabilities	(13,317)	(13,596)
Noncurrent liabilities	(66,648)	(65,932)
Net obligation recognized	(78,528)	(78,099)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial gain	(8,505)	(12,656)
Net amount recognized (before tax effect)	$ (8,505)	$ (12,656)