Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment			2
Segment Reporting Information [Line Items]
Revenues	$ 619,467	$ 633,180	$ 1,744,872	$ 1,800,824
Adjusted EBITDA	106,621	124,894	319,439	315,192
Depreciation, depletion and amortization	30,402	31,775	82,199	92,027
Accretion on asset retirement obligations	5,137	6,992	15,411	20,977
Total assets	1,934,600	1,997,622	1,934,600	1,997,622	$ 1,887,060
Capital expenditures	49,452	25,693	137,396	55,742
Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	269,967	261,927	692,845	737,233
Adjusted EBITDA	50,153	48,646	85,433	102,639
Depreciation, depletion and amortization	5,956	9,114	15,702	25,841
Accretion on asset retirement obligations	3,135	4,885	9,406	14,656
Total assets	236,656	374,092	236,656	374,092
Capital expenditures	5,402	3,458	19,026	7,221
Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	254,493	236,328	769,000	733,707
Adjusted EBITDA	70,814	81,250	264,284	251,649
Depreciation, depletion and amortization	19,962	18,106	53,687	53,109
Accretion on asset retirement obligations	531	469	1,592	1,406
Total assets	609,378	561,989	609,378	561,989
Capital expenditures	36,475	17,827	98,849	35,555
Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	94,052	130,663	278,235	321,997
Adjusted EBITDA	16,659	25,200	33,699	52,710
Depreciation, depletion and amortization	3,852	3,924	10,976	11,459
Accretion on asset retirement obligations	603	565	1,810	1,696
Total assets	148,994	127,904	148,994	127,904
Capital expenditures	6,837	3,332	16,299	7,097
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	955	4,262	4,792	7,887
Adjusted EBITDA	(31,005)	(30,202)	(63,977)	(91,806)
Depreciation, depletion and amortization	632	631	1,834	1,618
Accretion on asset retirement obligations	868	1,073	2,603	3,219
Total assets	939,572	933,637	939,572	933,637
Capital expenditures	$ 738	$ 1,076	$ 3,222	$ 5,869