Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			2
Revenues	$ 382,261	$ 619,467	$ 1,107,014	$ 1,744,872
Adjusted EBITDA	17,426	106,621	19,609	319,439
Depreciation, depletion and amortization	32,630	30,249	94,105	82,122
Accretion on asset retirement obligations	4,947	5,137	14,939	15,411
Total assets	1,653,337	1,934,600	1,653,337	1,934,600	$ 1,867,756
Capital expenditures	57,100	49,452	205,661	137,396
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	180,850	269,968	492,406	692,845
Adjusted EBITDA	34,486	50,153	28,542	85,433
Depreciation, depletion and amortization	5,847	5,956	16,337	15,702
Accretion on asset retirement obligations	3,495	3,135	10,484	9,406
Total assets	189,942	236,656	189,942	236,656
Capital expenditures	26	5,402	4,268	19,026
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	168,054	254,493	489,660	769,000
Adjusted EBITDA	12,407	70,814	76,037	264,284
Depreciation, depletion and amortization	24,221	19,962	69,028	53,687
Accretion on asset retirement obligations	486	531	1,458	1,592
Total assets	759,202	609,378	759,202	609,378
Capital expenditures	57,424	36,475	193,586	98,849
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	32,449	94,052	105,481	278,235
Adjusted EBITDA	(2,870)	16,659	(8,942)	33,699
Depreciation, depletion and amortization	2,303	3,852	6,973	10,976
Accretion on asset retirement obligations	347	603	1,042	1,810
Total assets	26,766	148,994	26,766	148,994
Capital expenditures	542	6,837	5,113	16,299
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	908	954	19,467	4,792
Adjusted EBITDA	(26,597)	(31,005)	(76,028)	(63,977)
Depreciation, depletion and amortization	259	479	1,767	1,757
Accretion on asset retirement obligations	619	868	1,955	2,603
Total assets	677,427	939,572	677,427	939,572
Capital expenditures	$ (892)	$ 738	$ 2,694	$ 3,222