Segment Information (Reconciliation Segment Income To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Information
Net income (loss)	$ (25,299)	$ 72,741
Provision for (benefit from) income taxes	(1,791)	70
Interest expense, net	2,129	2,289
Depreciation, depletion and amortization	31,308	25,273
Accretion on asset retirement obligations	5,006	5,137
Amortization of sales contracts, net		65
Costs related to proposed joint venture with Peabody Energy	3,664
Severance costs related to voluntary separation plan	5,828
Property insurance recovery related to Mountain Laurel longwall	9,000
Non-service related pension and postretirement benefit costs	1,096	1,766
Reorganization items, net	(26)	(87)
Adjusted EBITDA	$ 12,915	$ 107,254