Segment Information (Reconciliation Statement Of Segment Income from Operations To Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting [Abstract]
Net income	$ 123,192	$ 68,351	$ 226,483	$ 157,179
Benefit from income taxes	(45,215)	(1,643)	(49,125)	(484)
Interest expense, net	3,378	5,252	10,998	19,311
Depreciation, depletion and amortization	31,775	31,914	92,027	94,536
Accretion on asset retirement obligations	6,992	7,580	20,977	22,826
Amortization of sales contracts, net	3,241	13,861	9,540	42,903
Gain on sale of Lone Mountain Processing, Inc.	0	(21,574)	0	(21,574)
Net loss resulting from early retirement of debt and debt restructuring	0	486	485	2,547
Non-service related pension and postretirement benefit costs	971	821	2,206	1,774
Reorganization items, net	560	43	1,601	2,892
Adjusted EBITDA	$ 124,894	$ 105,091	$ 315,192	$ 321,910