Segment Information (Reconciliation Segment Income To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting [Abstract]
Net income	$ 106,769	$ 123,192	$ 242,350	$ 226,483
Provision for (benefit from) income taxes	347	(45,215)	508	(49,125)
Interest expense, net	340	3,378	4,916	10,998
Depreciation, depletion and amortization	30,402	31,775	82,199	92,027
Accretion on asset retirement obligations	5,137	6,992	15,411	20,977
Amortization of sales contracts, net	(153)	3,241	(77)	9,540
Loss on sale of Lone Mountain Processing, Inc.	0	0	4,304	0
Preference Rights Lease Application settlement income	(39,000)	0	(39,000)	0
Net loss resulting from early retirement of debt and debt restructuring	0	0	0	485
Non-service related pension and postretirement benefit costs	(975)	971	2,127	2,206
Reorganization items, net	0	560	(71)	1,601
Costs associated with proposed joint venture with Peabody Energy, Inc.	3,754	0	6,772	0
Adjusted EBITDA	$ 106,621	$ 124,894	$ 319,439	$ 315,192