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, 2018	Dec. 31, 2017	Dec. 31, 2016
CWP
Change in benefit obligation:
Benefit obligation at beginning of period	$ 162,840	$ 118,836
State administrative fees and insurance bond premiums	0	0	$ 0
Service cost	6,650	5,122	4,327
Interest cost	5,245	4,050	4,283
Actuarial loss (gain)	14,832	47,939
Benefits paid	(12,379)	(13,107)
Benefit obligation at end of period	177,188	162,840	118,836
Current assets	0	0
Current liabilities	(12,187)	(12,972)
Noncurrent liabilities	(165,001)	(149,868)
Net obligation recognized	(177,188)	(162,840)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial loss (gain)	8,542	(7,144)
Net amount recognized (before tax effect)	8,542	(7,144)
Workers' Compensation
Change in benefit obligation:
Benefit obligation at beginning of period	78,528	78,099
State administrative fees and insurance bond premiums	2,671	3,198	3,199
Service cost	6,230	5,734	7,466
Interest cost	2,283	2,321	2,499
Actuarial loss (gain)	(5,134)	3,553
Benefits paid	(13,592)	(14,377)
Benefit obligation at end of period	70,986	78,528	$ 78,099
Current assets	1,384	1,437
Current liabilities	(12,628)	(13,317)
Noncurrent liabilities	(59,742)	(66,648)
Net obligation recognized	(70,986)	(78,528)
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]
Net actuarial loss (gain)	(13,561)	(8,505)
Net amount recognized (before tax effect)	$ (13,561)	$ (8,505)