Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment	2
Revenues	$ 1,467,592	$ 2,294,352	$ 2,451,787
Adjusted EBITDA	23,743	363,167	437,778
Depreciation, depletion and amortization	121,552	111,621	130,670
Accretion on asset retirement obligations	19,887	20,548	27,970
Total assets	1,722,472	1,867,756	1,887,060
Capital expenditures	285,821	266,356	95,272
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	662,135	915,750	973,248
Adjusted EBITDA	50,246	110,528	126,525
Depreciation, depletion and amortization	21,375	20,810	33,120
Accretion on asset retirement obligations	13,979	12,542	19,541
Total assets	179,401	256,460	278,314
Capital expenditures	4,688	29,420	12,140
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	641,536	990,550	1,036,621
Adjusted EBITDA	91,322	305,363	349,524
Depreciation, depletion and amortization	91,202	74,211	69,560
Accretion on asset retirement obligations	1,943	2,123	1,874
Total assets	811,605	625,134	545,061
Capital expenditures	269,273	211,718	64,307
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	139,497	377,202	428,884
Adjusted EBITDA	(16,211)	41,495	68,620
Depreciation, depletion and amortization	6,976	14,414	14,699
Accretion on asset retirement obligations	1,389	2,413	2,261
Total assets	16,935	105,411	125,333
Capital expenditures	6,031	20,088	11,999
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	24,424	10,850	13,034
Adjusted EBITDA	(101,614)	(94,219)	(106,891)
Depreciation, depletion and amortization	1,999	2,186	13,291
Accretion on asset retirement obligations	2,576	3,470	4,294
Total assets	714,531	880,751	938,352
Capital expenditures	$ 5,829	$ 5,130	$ 6,826