Segment Information (Reconciliation Segment Income To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Net income (loss)	$ (191,467)	$ 106,769	$ (266,090)	$ 242,350
Provision for (benefit from) income taxes	379	347	(206)	508
Interest expense, net	2,530	340	6,389	4,916
Depreciation, depletion and amortization	32,630	30,249	94,105	82,122
Accretion on asset retirement obligations	4,947	5,137	14,939	15,411
Costs related to proposed joint venture with Peabody Energy	4,423	3,754	15,938	6,772
Asset impairment	163,088		163,088
Severance costs related to voluntary separation plan	18		13,283
Gain on property insurance recovery related to Mountain Laurel longwall		39,000	23,518	39,000
(Gain) loss on divestitures			(1,369)	4,304
Preference Rights Lease Application settlement income		(39,000)	(23,518)	(39,000)
Non-service related pension and postretirement benefit costs	878	(975)	3,076	2,127
Reorganization items, net			(26)	(71)
Adjusted EBITDA	$ 17,426	$ 106,621	19,609	319,439
Dal-Tex and Briar Branch Properties
Segment Reporting Information [Line Items]
(Gain) loss on divestitures			$ (1,369)	$ 4,304