Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) Segment	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment	2
Segment Reporting Information [Line Items]
Revenues	$ 555,183	$ 575,295
Adjusted EBITDA	107,254	104,913
Depreciation, depletion and amortization	25,273	29,703
Accretion on asset retirement obligations	5,137	6,992
Total assets	1,863,201	1,976,678	$ 1,887,060
Capital expenditures	39,147	9,453
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	212,729	245,428
Adjusted EBITDA	20,583	27,502
Depreciation, depletion and amortization	4,865	8,423
Accretion on asset retirement obligations	3,135	4,885
Total assets	230,329	383,823
Capital expenditures	414	698
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	253,262	238,347
Adjusted EBITDA	91,534	83,742
Depreciation, depletion and amortization	16,382	16,986
Accretion on asset retirement obligations	531	469
Total assets	568,367	548,804
Capital expenditures	31,224	5,829
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	85,978	91,520
Adjusted EBITDA	6,119	15,669
Depreciation, depletion and amortization	3,435	3,835
Accretion on asset retirement obligations	603	565
Total assets	139,306	130,132
Capital expenditures	6,250	1,206
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	3,214	0
Adjusted EBITDA	(10,982)	(22,000)
Depreciation, depletion and amortization	591	459
Accretion on asset retirement obligations	868	1,073
Total assets	925,199	913,919
Capital expenditures	$ 1,259	$ 1,720