Segment Information (Schedule of Operating Segment Results) (Details) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Oct. 01, 2016	Dec. 31, 2017	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues					$ 575,688					$ 2,324,623
Depreciation, depletion and amortization					32,604					122,464
Accretion on asset retirement obligations					7,634					30,209
Assets	$ 1,979,632				2,136,597					1,979,632
Capital expenditures					15,214					59,205
Successor
Segment Reporting Information [Line Items]
Revenues	560,244				575,688					2,324,623
Adjusted EBITDAR					94,497					417,757
Depreciation, depletion and amortization					32,604					122,464
Accretion on asset retirement obligations					7,634					30,209
Assets					2,136,597
Capital expenditures					15,214					59,205
Successor | Operating segments | PRB
Segment Reporting Information [Line Items]
Revenues					275,703					1,024,197
Adjusted EBITDAR					55,765					158,882
Depreciation, depletion and amortization					9,949					36,349
Accretion on asset retirement obligations					5,049					20,160
Assets	390,665				446,775					390,665
Capital expenditures					934					6,212
Successor | Operating segments | MET
Segment Reporting Information [Line Items]
Revenues					200,377					887,839
Adjusted EBITDAR					30,819					243,616
Depreciation, depletion and amortization					18,287					70,896
Accretion on asset retirement obligations					528					2,000
Assets	548,476				576,793					548,476
Capital expenditures					13,329					32,678
Successor | Operating segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues					97,382					396,504
Adjusted EBITDAR					31,159					102,006
Depreciation, depletion and amortization					3,911					13,588
Accretion on asset retirement obligations					540					2,161
Assets	134,397				129,602					134,397
Capital expenditures					684					11,901
Successor | Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues					2,226					16,083
Adjusted EBITDAR					(23,246)					(86,747)
Depreciation, depletion and amortization					457					1,631
Accretion on asset retirement obligations					1,517					5,888
Assets	$ 906,094				983,427					906,094
Capital expenditures					$ 267					$ 8,414
Predecessor
Segment Reporting Information [Line Items]
Revenues		$ 613,538	$ 549,866	$ 600,975		$ 550,305	$ 420,298	$ 428,106	$ 1,398,709		$ 2,573,260
Adjusted EBITDAR									87,303		283,797
Depreciation, depletion and amortization									191,581		379,345
Accretion on asset retirement obligations									24,321		33,680
Assets									2,123,829		5,041,881
Capital expenditures									82,434		119,024
Predecessor | Operating segments | PRB
Segment Reporting Information [Line Items]
Revenues									726,747		1,448,440
Adjusted EBITDAR									113,185		281,039
Depreciation, depletion and amortization									100,151		176,257
Accretion on asset retirement obligations									16,940		22,156
Assets									456,711		1,648,916
Capital expenditures									612		21,228
Predecessor | Operating segments | MET
Segment Reporting Information [Line Items]
Revenues									437,069		637,941
Adjusted EBITDAR									11,851		70,450
Depreciation, depletion and amortization									55,311		133,463
Accretion on asset retirement obligations									1,765		2,267
Assets									619,154		772,439
Capital expenditures									17,296		24,787
Predecessor | Operating segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues									213,052		428,809
Adjusted EBITDAR									31,448		42,734
Depreciation, depletion and amortization									32,310		47,786
Accretion on asset retirement obligations									1,988		2,658
Assets									131,173		366,610
Capital expenditures									3,895		11,277
Predecessor | Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues									21,841		58,070
Adjusted EBITDAR									(69,181)		(110,426)
Depreciation, depletion and amortization									3,809		21,839
Accretion on asset retirement obligations									3,628		6,599
Assets									916,791		2,253,916
Capital expenditures									$ 60,631		$ 61,732