Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment			2
Revenues	$ 319,521	$ 570,222	$ 724,753	$ 1,125,405
Adjusted EBITDA	(10,732)	105,564	2,183	212,818
Depreciation, depletion and amortization	30,167	26,535	61,475	51,873
Accretion on asset retirement obligations	4,986	5,137	9,992	10,274
Total assets	1,802,389	1,889,530	1,802,389	1,889,530	$ 1,867,756
Capital expenditures	60,872	48,708	148,561	87,854
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	133,096	210,149	311,556	422,878
Adjusted EBITDA	(5,362)	14,696	(5,944)	35,279
Depreciation, depletion and amortization	5,283	4,880	10,491	9,745
Accretion on asset retirement obligations	3,495	3,135	6,990	6,271
Total assets	247,990	236,527	247,990	236,527
Capital expenditures	1,145	13,209	4,242	13,623
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	138,951	261,245	321,605	514,507
Adjusted EBITDA	20,910	101,936	63,630	193,470
Depreciation, depletion and amortization	22,289	17,343	44,807	33,725
Accretion on asset retirement obligations	486	531	972	1,061
Total assets	740,451	605,657	740,451	605,657
Capital expenditures	57,514	31,150	136,162	62,374
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	41,297	98,205	73,033	184,183
Adjusted EBITDA	(4,752)	10,922	(6,072)	17,041
Depreciation, depletion and amortization	2,333	3,689	4,670	7,124
Accretion on asset retirement obligations	347	603	695	1,207
Total assets	108,238	136,899	108,238	136,899
Capital expenditures	955	3,211	4,571	9,461
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	6,177	623	18,559	3,837
Adjusted EBITDA	(21,528)	(21,990)	(49,431)	(32,972)
Depreciation, depletion and amortization	262	623	1,507	1,279
Accretion on asset retirement obligations	658	868	1,335	1,735
Total assets	705,710	910,447	705,710	910,447
Capital expenditures	$ 1,258	$ 1,138	$ 3,586	$ 2,396