Segment Information (Details) - USD ($) $ in Millions			3 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended
		Apr. 01, 2017	Sep. 30, 2017	Apr. 01, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Asset impairment								$ 247.9
DTA and PBGC
Reportable segment results
Net gain on disposal of assets				$ 19.7
Successor
Reportable segment results
Revenues			$ 1,477.2			$ 2,735.5
Total Adjusted EBITDA			411.3			729.1
Gain on VEBA settlement						0.0
Net gain on disposal of assets			0.4			0.9
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income (loss) from continuing operations, net of income taxes			233.7			335.1
Depreciation, depletion and amortization			194.5			342.8
Asset retirement obligation expenses			11.3			22.3
Selling and administrative expenses related to debt restructuring			0.0			0.0
Asset impairment			0.0			0.0
Change in deferred tax asset valuation allowance related to equity affiliates			(3.4)			(7.7)
Interest expense			42.4			83.8
Loss on Extinguishment of Debt			12.9			12.9
Interest income			(2.0)			(3.5)
Break fees related to terminated asset sales			0.0			(28.0)
Unrealized losses (gains) on non-coal trading derivative contracts			1.7			(1.5)
Unrealized losses (gains) on economic hedges			10.8			1.4
Coal inventory revaluation			0.0			67.3
Take-or-pay contract-based intangible recognition			(6.5)			(16.4)
Reorganization items, net			0.0			0.0
Income tax provision (benefit)			(84.1)			(79.4)
Total Adjusted EBITDA			411.3			729.1
Predecessor
Reportable segment results
Revenues				1,326.2	$ 1,207.1		$ 3,274.5
Total Adjusted EBITDA				341.3	130.2		238.0
Gain on VEBA settlement				0.0			68.1
Net gain on disposal of assets				22.8	1.9		17.4
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income (loss) from continuing operations, net of income taxes				(195.5)	(97.7)		(488.6)
Depreciation, depletion and amortization				119.9	117.8		345.5
Asset retirement obligation expenses				14.6	12.7		37.3
Selling and administrative expenses related to debt restructuring				0.0	0.0		21.5
Asset impairment				30.5	0.0		17.2
Change in deferred tax asset valuation allowance related to equity affiliates				(5.2)	(0.6)		(0.6)
Interest expense				32.9	58.5		243.7
Loss on Extinguishment of Debt				0.0	0.0		0.0
Interest income				(2.7)	(1.3)		(4.0)
Break fees related to terminated asset sales				0.0	0.0		0.0
Unrealized losses (gains) on non-coal trading derivative contracts				0.0	0.0		0.0
Unrealized losses (gains) on economic hedges				(16.6)	21.9		49.1
Coal inventory revaluation				0.0	0.0		0.0
Take-or-pay contract-based intangible recognition				0.0	0.0		0.0
Reorganization items, net				627.2	29.7		125.1
Income tax provision (benefit)		$ (266.0)		(263.8)	(10.8)		(108.2)
Total Adjusted EBITDA				341.3	130.2		238.0
Operating Segments | Successor | Powder River Basin Mining
Reportable segment results
Revenues			420.9			786.3
Total Adjusted EBITDA			112.7			197.5
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			112.7			197.5
Operating Segments | Successor | Midwestern U.S. Mining
Reportable segment results
Revenues			207.7			402.6
Total Adjusted EBITDA			49.5			96.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			49.5			96.0
Operating Segments | Successor | Western U.S. Mining
Reportable segment results
Revenues			155.7			281.1
Total Adjusted EBITDA			34.5			79.4
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			34.5			79.4
Operating Segments | Successor | Australian Metallurgical Mining
Reportable segment results
Revenues			415.9			703.7
Total Adjusted EBITDA			143.1			215.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			143.1			215.0
Operating Segments | Successor | Australian Thermal Mining
Reportable segment results
Revenues			265.8			505.0
Total Adjusted EBITDA			97.8			203.7
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			97.8			203.7
Operating Segments | Predecessor | Powder River Basin Mining
Reportable segment results
Revenues				394.3	419.6		1,062.2
Total Adjusted EBITDA				91.7	123.9		278.3
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA				91.7	123.9		278.3
Operating Segments | Predecessor | Midwestern U.S. Mining
Reportable segment results
Revenues				193.2	211.0		599.6
Total Adjusted EBITDA				50.0	59.1		172.4
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA				50.0	59.1		172.4
Operating Segments | Predecessor | Western U.S. Mining
Reportable segment results
Revenues				149.7	162.4		387.0
Total Adjusted EBITDA				50.0	34.3		83.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA				50.0	34.3		83.2
Operating Segments | Predecessor | Australian Metallurgical Mining
Reportable segment results
Revenues				328.9	232.5		682.8
Total Adjusted EBITDA				109.6	(34.5)		(121.0)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA				109.6	(34.5)		(121.0)
Operating Segments | Predecessor | Australian Thermal Mining
Reportable segment results
Revenues				224.8	197.9		561.4
Total Adjusted EBITDA				75.6	48.9		137.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA				75.6	48.9		137.2
Corporate, Non-Segment | Successor | Trading and Brokerage
Reportable segment results
Revenues			19.4			24.6
Total Adjusted EBITDA			2.7			(2.4)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			2.7			(2.4)
Corporate, Non-Segment | Successor | Corporate and Other
Reportable segment results
Revenues			(8.2)			32.2
Total Adjusted EBITDA	[1]		(29.0)			(60.1)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA	[1]		$ (29.0)			$ (60.1)
Corporate, Non-Segment | Predecessor | Trading and Brokerage
Reportable segment results
Revenues				15.0	2.7		16.5
Total Adjusted EBITDA				8.8	(9.4)		(41.3)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA				8.8	(9.4)		(41.3)
Corporate, Non-Segment | Predecessor | Corporate and Other
Reportable segment results
Revenues				20.3	(19.0)		(35.0)
Total Adjusted EBITDA	[1]			(44.4)	(92.1)		(270.8)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA	[1]			$ (44.4)	$ (92.1)		(270.8)
Funding of VEBA Pursuant to Settlement with UMWA and Patriot Coal Corporation | Spinoff | Patriot
Reportable segment results
Gain on VEBA settlement							$ 68.1

[1] Includes a gain of $19.7 million related to the sale of Dominion Terminal Associates during the predecessor period January 1 through April 1, 2017 and a gain of $68.1 million related to the 2016 Settlement Agreement described in Note 5. “Discontinued Operations” during the predecessor nine months ended September 30, 2016.