Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment			2
Segment Reporting Information [Line Items]
Revenues	$ 633,180	$ 613,538	$ 1,800,824	$ 1,764,379
Adjusted EBITDA	124,894	105,091	315,192	321,910
Depreciation, depletion and amortization	31,775	31,914	92,027	94,536
Accretion on asset retirement obligations	6,992	7,580	20,977	22,826
Total assets	1,997,622	2,022,533	1,997,622	2,022,533	$ 1,979,632
Capital expenditures	25,693	13,581	55,742	30,503
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	261,927	276,000	737,233	780,007
Adjusted EBITDA	48,646	48,768	102,639	128,562
Depreciation, depletion and amortization	9,114	9,577	25,841	27,661
Accretion on asset retirement obligations	4,885	5,040	14,656	15,120
Total assets	374,092	419,162	374,092	419,162
Capital expenditures	3,458	2,047	7,221	2,997
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	236,328	238,946	733,707	692,178
Adjusted EBITDA	81,250	53,346	251,649	184,208
Depreciation, depletion and amortization	18,106	18,479	53,109	55,629
Accretion on asset retirement obligations	469	511	1,406	1,568
Total assets	561,989	538,637	561,989	538,637
Capital expenditures	17,827	4,597	35,555	16,032
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	130,663	93,859	321,997	287,404
Adjusted EBITDA	25,200	21,217	52,710	75,369
Depreciation, depletion and amortization	3,924	3,465	11,459	9,950
Accretion on asset retirement obligations	565	540	1,696	1,621
Total assets	127,904	130,729	127,904	130,729
Capital expenditures	3,332	4,013	7,097	6,653
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	4,262	4,733	7,887	4,790
Adjusted EBITDA	(30,202)	(18,240)	(91,806)	(66,229)
Depreciation, depletion and amortization	631	393	1,618	1,296
Accretion on asset retirement obligations	1,073	1,489	3,219	4,517
Total assets	933,637	934,005	933,637	934,005
Capital expenditures	$ 1,076	$ 2,924	$ 5,869	$ 4,821