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, 2019	Dec. 31, 2018	Dec. 31, 2017
CWP
Change in benefit obligation:
Benefit obligation at beginning of period	$ 177,188	$ 162,840
State administrative fees and insurance bond premiums	0	0	$ 0
Service cost	3,791	6,650	5,122
Interest cost	7,001	5,245	4,050
Actuarial loss (gain)	39,827	14,832
Benefits paid	(13,334)	(12,379)
Benefit obligation at end of period	214,473	177,188	162,840
Current assets	0	0
Current liabilities	(12,331)	(12,187)
Noncurrent liabilities	(202,142)	(165,001)
Net obligation recognized	(214,473)	(177,188)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss (gain)	47,352	8,542
Net amount recognized (before tax effect)	47,352	8,542
Workers' Compensation
Change in benefit obligation:
Benefit obligation at beginning of period	70,986	78,528
State administrative fees and insurance bond premiums	2,157	2,671	3,198
Service cost	5,685	6,230	5,734
Interest cost	2,585	2,283	2,321
Actuarial loss (gain)	1,536	(5,134)
Benefits paid	(11,469)	(13,592)
Benefit obligation at end of period	71,480	70,986	$ 78,528
Current assets	1,037	1,384
Current liabilities	(11,323)	(12,628)
Noncurrent liabilities	(61,194)	(59,742)
Net obligation recognized	(71,480)	(70,986)
Amounts recognized in accumulated other comprehensive loss consist of:
Net actuarial loss (gain)	(11,250)	(13,561)
Net amount recognized (before tax effect)	$ (11,250)	$ (13,561)