Segment Information (Reconciliation Segment Income To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting [Abstract]
Net income	$ 62,840	$ 43,306	$ 135,581	$ 103,291
Provision for (benefit from) income taxes	91	(3,366)	161	(3,910)
Interest expense, net	2,287	3,498	4,576	7,620
Depreciation, depletion and amortization	26,524	30,549	51,797	60,252
Accretion on asset retirement obligations	5,137	6,993	10,274	13,985
Amortization of sales contracts, net	11	3,248	76	6,299
Loss on sale of Lone Mountain Processing, Inc.	4,304	0	4,304	0
Net loss resulting from early retirement of debt and debt restructuring	0	485	0	485
Non-service related pension and postretirement benefit costs	1,336	(68)	3,102	1,235
Reorganization items, net	16	740	(71)	1,041
Costs associated with proposed joint venture with Peabody Energy, Inc.	3,018	0	3,018	0
Adjusted EBITDA	$ 105,564	$ 85,385	$ 212,818	$ 190,298