Segment Information (Reconciliation Segment Income To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net income (loss)	$ (344,615)	$ 233,799	$ 312,577
Provision for (benefit from) income taxes	(7)	248	(52,476)
Interest expense, net	10,624	6,794	13,689
Depreciation, depletion and amortization	121,552	111,621	130,670
Accretion on asset retirement obligations	19,887	20,548	27,970
Costs related to proposed joint venture with Peabody Energy	16,087	13,816
Asset impairment and restructuring	221,380
Severance costs related to voluntary separation plan	13,373
Gain on property insurance recovery related to Mountain Laurel longwall	(23,518)	0
(Gain) loss on divestitures	(1,505)	13,312
Preference Rights Lease Application settlement income	0	(39,000)
Net loss resulting from early retirement of debt and debt restructuring			485
Non-service related pension and postretirement benefit costs	3,884	2,053	3,202
Reorganization items, net	(26)	(24)	1,661
Adjusted EBITDA	23,743	363,167	437,778
EBITDA from idled or otherwise disposed operations	15,858	12,926	2,492
Selling, general and administrative expenses	82,397	95,781	100,300
Other	3,359	(14,488)	4,099
Segment Adjusted EBITDA from coal operations	125,357	457,386	$ 544,669
Dal-Tex and Briar Branch Properties
Segment Reporting Information [Line Items]
(Gain) loss on divestitures	$ (1,505)	$ 13,312