Segment Information (Schedule of Operating Segment Results) (Details) - USD ($) $ in Thousands	3 Months Ended									9 Months Ended	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Oct. 01, 2016	Dec. 31, 2018	Dec. 31, 2017	Oct. 02, 2016
Segment Reporting Information [Line Items]
Revenues	$ 650,963	$ 633,180	$ 592,349	$ 575,295	$ 560,244	$ 613,538	$ 549,866	$ 600,975	$ 575,688		$ 2,451,787	$ 2,324,623
Adjusted EBITDAR									94,465		437,778	419,697
Depreciation, depletion and amortization									32,604		119,563	122,464
Accretion on asset retirement obligations									7,634		27,970	30,209
Total Assets	1,887,060				1,979,632						1,887,060	1,979,632	$ 2,136,597
Capital expenditures									15,214		95,272	59,205
Operating segments | PRB
Segment Reporting Information [Line Items]
Revenues									275,703		973,248	1,024,197
Adjusted EBITDAR									55,765		126,525	158,882
Depreciation, depletion and amortization									9,949		33,120	36,349
Accretion on asset retirement obligations									5,049		19,541	20,160
Total Assets	278,314				390,665						278,314	390,665	446,775
Capital expenditures									934		12,140	6,212
Operating segments | MET
Segment Reporting Information [Line Items]
Revenues									200,377		1,036,621	887,839
Adjusted EBITDAR									30,819		349,524	243,616
Depreciation, depletion and amortization									18,287		69,560	70,896
Accretion on asset retirement obligations									528		1,874	2,000
Total Assets	545,061				548,476						545,061	548,476	576,793
Capital expenditures									13,329		64,307	32,678
Operating segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues									97,382		428,884	396,504
Adjusted EBITDAR									31,159		68,620	102,006
Depreciation, depletion and amortization									3,911		14,699	13,588
Accretion on asset retirement obligations									540		2,261	2,161
Total Assets	125,333				134,397						125,333	134,397	129,602
Capital expenditures									684		11,999	11,901
Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues									2,226		13,034	16,083
Adjusted EBITDAR									(23,278)		(106,891)	(84,807)
Depreciation, depletion and amortization									457		2,184	1,631
Accretion on asset retirement obligations									1,517		4,294	5,888
Total Assets	$ 938,352				$ 906,094						938,352	906,094	$ 983,427
Capital expenditures									$ 267		$ 6,826	$ 8,414
Predecessor
Segment Reporting Information [Line Items]
Revenues										$ 1,398,709
Adjusted EBITDAR										89,018
Depreciation, depletion and amortization										191,581
Accretion on asset retirement obligations										24,321
Total Assets										2,123,829
Capital expenditures										82,434
Predecessor | Operating segments | PRB
Segment Reporting Information [Line Items]
Revenues										726,747
Adjusted EBITDAR										113,185
Depreciation, depletion and amortization										100,151
Accretion on asset retirement obligations										16,940
Total Assets										456,711
Capital expenditures										612
Predecessor | Operating segments | MET
Segment Reporting Information [Line Items]
Revenues										437,069
Adjusted EBITDAR										11,851
Depreciation, depletion and amortization										55,311
Accretion on asset retirement obligations										1,765
Total Assets										619,154
Capital expenditures										17,296
Predecessor | Operating segments | Other Thermal
Segment Reporting Information [Line Items]
Revenues										213,052
Adjusted EBITDAR										31,448
Depreciation, depletion and amortization										32,310
Accretion on asset retirement obligations										1,988
Total Assets										131,173
Capital expenditures										3,895
Predecessor | Corporate, Other and Eliminations
Segment Reporting Information [Line Items]
Revenues										21,841
Adjusted EBITDAR										(67,466)
Depreciation, depletion and amortization										3,809
Accretion on asset retirement obligations										3,628
Total Assets										916,791
Capital expenditures										$ 60,631