Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) Segment	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment			2
Segment Reporting Information [Line Items]
Revenues	$ 592,349	$ 549,866	$ 1,167,644	$ 1,150,841
Adjusted EBITDA	85,385	95,600	190,298	216,819
Depreciation, depletion and amortization	30,549	30,701	60,252	62,622
Accretion on asset retirement obligations	6,993	7,623	13,985	15,246
Total assets	1,949,413	2,132,058	1,949,413	2,132,058	$ 1,979,632
Capital expenditures	20,596	10,972	30,049	16,922
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	229,878	230,579	475,306	504,007
Adjusted EBITDA	26,491	31,789	53,993	79,794
Depreciation, depletion and amortization	8,304	8,574	16,727	18,085
Accretion on asset retirement obligations	4,885	5,040	9,771	10,080
Total assets	379,613	426,793	379,613	426,793
Capital expenditures	3,065	822	3,763	950
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	259,032	227,649	497,379	453,232
Adjusted EBITDA	86,657	62,552	170,399	130,862
Depreciation, depletion and amortization	18,018	18,385	35,003	37,149
Accretion on asset retirement obligations	469	528	937	1,057
Total assets	551,012	580,543	551,012	580,543
Capital expenditures	11,899	6,825	17,728	11,436
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	99,814	91,639	191,334	193,545
Adjusted EBITDA	11,842	26,910	27,510	54,152
Depreciation, depletion and amortization	3,701	3,285	7,536	6,485
Accretion on asset retirement obligations	565	540	1,130	1,080
Total assets	134,319	126,870	134,319	126,870
Capital expenditures	2,559	1,899	3,765	2,640
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	3,625	(1)	3,625	57
Adjusted EBITDA	(39,605)	(25,651)	(61,604)	(47,989)
Depreciation, depletion and amortization	526	457	986	903
Accretion on asset retirement obligations	1,074	1,515	2,147	3,029
Total assets	884,469	997,852	884,469	997,852
Capital expenditures	$ 3,073	$ 1,426	$ 4,793	$ 1,896