Segment Information (Reconciliation Segment Income To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Information
Net income (loss)	$ 89,143	$ (191,467)	$ 110,967	$ (266,090)
Provision for (benefit from) income taxes	(1,082)	379	1,301	(206)
Interest expense, net	6,151	2,530	12,746	6,389
Depreciation, depletion and amortization	30,760	32,630	84,441	94,105
Accretion on asset retirement obligations	5,437	4,947	16,311	14,939
Costs related to proposed joint venture with Peabody Energy		4,423		15,938
Asset impairment and restructuring		163,106		176,371
Gain on property insurance recovery related to Mountain Laurel longwall				(23,518)
Gain on divestitures				(1,369)
Non-service related pension and postretirement benefit costs	1,186	878	3,252	3,076
Reorganization items, net				(26)
Adjusted EBITDA	131,595	17,426	229,018	19,609
EBITDA from idled or otherwise disposed operations	3,074	2,896	10,637	10,691
Selling, general and administrative expenses	21,081	21,541	66,679	64,024
Other	15,535	2,160	22,646	1,313
Segment Adjusted EBITDA from coal operations	$ 171,285	$ 44,023	$ 328,980	$ 95,637