Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Information
Number of reportable segments | segment	2
Segment Reporting Information [Line Items]
Revenues	$ 405,232	$ 555,183
Adjusted EBITDA	12,915	107,254
Depreciation, depletion and amortization	31,308	25,273
Accretion on asset retirement obligations	5,006	5,137
Total assets	1,857,506	1,863,201	$ 1,867,756
Capital expenditures	87,690	39,147
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	178,460	212,729
Adjusted EBITDA	(582)	20,583
Depreciation, depletion and amortization	5,208	4,865
Accretion on asset retirement obligations	3,495	3,135
Total assets	256,937	230,329
Capital expenditures	3,097	414
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	182,654	253,262
Adjusted EBITDA	42,720	91,534
Depreciation, depletion and amortization	22,517	16,382
Accretion on asset retirement obligations	486	531
Total assets	693,227	568,367
Capital expenditures	78,648	31,224
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	31,736	85,978
Adjusted EBITDA	(1,320)	6,119
Depreciation, depletion and amortization	2,337	3,435
Accretion on asset retirement obligations	347	603
Total assets	120,869	139,306
Capital expenditures	3,616	6,250
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	12,382	3,214
Adjusted EBITDA	(27,903)	(10,982)
Depreciation, depletion and amortization	1,246	591
Accretion on asset retirement obligations	678	868
Total assets	786,473	925,199
Capital expenditures	$ 2,329	$ 1,259