Segment Information (Schedule Of Operating Segment Results) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Segment	Sep. 30, 2016 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | Segment			2
Successor
Segment Reporting Information [Line Items]
Revenues	$ 613,538		$ 1,764,379
Adjusted EBITDAR	104,270		320,136
Depreciation, depletion and amortization	31,914		94,536
Accretion on asset retirement obligations	7,580		22,826
Capital expenditures	13,581		30,503
Successor | Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues	275,999		780,006
Adjusted EBITDAR	48,768		128,562
Depreciation, depletion and amortization	9,577		27,661
Accretion on asset retirement obligations	5,040		15,120
Capital expenditures	2,047		2,997
Successor | Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues	238,946		692,178
Adjusted EBITDAR	53,346		184,208
Depreciation, depletion and amortization	18,479		55,629
Accretion on asset retirement obligations	511		1,568
Capital expenditures	4,597		16,032
Successor | Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues	93,859		287,404
Adjusted EBITDAR	21,217		75,369
Depreciation, depletion and amortization	3,465		9,950
Accretion on asset retirement obligations	540		1,621
Capital expenditures	4,013		6,653
Successor | Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues	4,734		4,791
Adjusted EBITDAR	(19,061)		(68,003)
Depreciation, depletion and amortization	393		1,296
Accretion on asset retirement obligations	1,489		4,517
Capital expenditures	$ 2,924		$ 4,821
Predecessor
Segment Reporting Information [Line Items]
Revenues		$ 550,305		$ 1,398,709
Adjusted EBITDAR		89,333		87,303
Depreciation, depletion and amortization		69,423		191,581
Accretion on asset retirement obligations		7,965		24,321
Capital expenditures		8,296		82,434
Predecessor | Operating Segments | PRB
Segment Reporting Information [Line Items]
Revenues		295,891		726,747
Adjusted EBITDAR		78,880		113,185
Depreciation, depletion and amortization		37,246		100,151
Accretion on asset retirement obligations		5,647		16,940
Capital expenditures		113		612
Predecessor | Operating Segments | MET
Segment Reporting Information [Line Items]
Revenues		152,986		437,069
Adjusted EBITDAR		9,092		11,851
Depreciation, depletion and amortization		18,636		55,311
Accretion on asset retirement obligations		588		1,765
Capital expenditures		6,966		17,296
Predecessor | Operating Segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues		96,889		213,052
Adjusted EBITDAR		28,704		31,448
Depreciation, depletion and amortization		12,513		32,310
Accretion on asset retirement obligations		663		1,988
Capital expenditures		1,001		3,895
Predecessor | Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues		4,539		21,841
Adjusted EBITDAR		(27,343)		(69,181)
Depreciation, depletion and amortization		1,028		3,809
Accretion on asset retirement obligations		1,067		3,628
Capital expenditures		$ 216		$ 60,631