Segment Information (Details) - USD ($) $ in Millions			3 Months Ended			6 Months Ended
		Apr. 01, 2017	Jun. 30, 2018	Jun. 30, 2017	Apr. 01, 2017	Jun. 30, 2018
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Loss on early debt extinguishment		$ 0.0
Successor
Reportable segment results
Revenues			$ 1,309.4	$ 1,258.3		$ 2,772.1
Total Adjusted EBITDA			369.6	317.8		733.5
Net gain on disposal of assets			(1.6)	0.5		29.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income (loss) from continuing operations, net of income taxes			120.0	101.4		328.3
Depreciation, depletion and amortization			163.9	148.3		333.5
Asset retirement obligation expenses			13.2	11.0		25.5
Asset impairment			0.0	0.0		0.0
Changes in deferred tax asset valuation allowance and amortization of basis difference related to equity affiliates			(8.4)	(4.3)		(16.0)
Interest expense			38.3	41.4		74.6
Loss on early debt extinguishment			2.0	0.0		2.0
Interest income			(7.0)	(1.5)		(14.2)
Reorganization items, net			0.0	0.0		(12.8)
Break Fees On Terminated Asset Sales			0.0	(28.0)		0.0
Unrealized losses (gains) on economic hedges			48.1	(9.4)		9.5
Unrealized (gains) losses on non-coal trading derivative contracts			(0.1)	(3.2)		1.7
Fresh-Start Adjustment, Revaluation of Inventory			0.0	67.3		0.0
Take-or-pay contract-based intangible recognition			(7.8)	(9.9)		(16.1)
Income tax provision (benefit)			7.4	4.7		17.5
Total Adjusted EBITDA			369.6	317.8		733.5
Successor | Red Mountain Joint Venture
Reportable segment results
Net gain on disposal of assets						7.1
Successor | Iffley Land Transaction [Member]
Reportable segment results
Net gain on disposal of assets						20.6
Successor | Powder River Basin Mining | Operating Segments
Reportable segment results
Revenues			321.5	365.4		710.8
Total Adjusted EBITDA			62.0	84.8		136.5
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			62.0	84.8		136.5
Successor | Midwestern U.S. Mining | Operating Segments
Reportable segment results
Revenues			197.5	194.9		399.2
Total Adjusted EBITDA			42.0	46.5		73.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			42.0	46.5		73.2
Successor | Western U.S. Mining | Operating Segments
Reportable segment results
Revenues			139.6	125.4		283.3
Total Adjusted EBITDA			33.9	44.9		65.9
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			33.9	44.9		65.9
Successor | Australian Metallurgical Mining | Operating Segments
Reportable segment results
Revenues			417.5	287.8		883.7
Total Adjusted EBITDA			158.5	71.9		324.9
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			158.5	71.9		324.9
Successor | Australian Thermal Mining | Operating Segments
Reportable segment results
Revenues			267.4	239.2		468.8
Total Adjusted EBITDA			107.6	105.9		169.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			107.6	105.9		169.2
Successor | Trading and Brokerage | Corporate, Non-Segment
Reportable segment results
Revenues			10.0	5.2		30.1
Total Adjusted EBITDA			3.1	(5.1)		4.3
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA			3.1	(5.1)		4.3
Successor | Corporate and Other | Corporate, Non-Segment
Reportable segment results
Revenues	[1]		(44.1)	40.4		(3.8)
Total Adjusted EBITDA	[2]		(37.5)	(31.1)		(40.5)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA	[2]		$ (37.5)	(31.1)		$ (40.5)
Predecessor
Reportable segment results
Revenues		0.0			$ 1,326.2
Total Adjusted EBITDA		0.0			341.3
Net gain on disposal of assets		0.0			22.8
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income (loss) from continuing operations, net of income taxes		(319.8)			(195.5)
Depreciation, depletion and amortization		0.0			119.9
Asset retirement obligation expenses		0.0			14.6
Asset impairment		0.0			30.5
Changes in deferred tax asset valuation allowance and amortization of basis difference related to equity affiliates		0.0			(5.2)
Interest expense		0.0			32.9
Loss on early debt extinguishment		0.0			0.0
Interest income		0.0			(2.7)
Reorganization items, net		585.8			627.2
Break Fees On Terminated Asset Sales		0.0			0.0
Unrealized losses (gains) on economic hedges		0.0			(16.6)
Unrealized (gains) losses on non-coal trading derivative contracts		0.0			0.0
Fresh-Start Adjustment, Revaluation of Inventory		0.0			0.0
Take-or-pay contract-based intangible recognition		0.0			0.0
Income tax provision (benefit)		(266.0)			(263.8)
Total Adjusted EBITDA		$ 0.0			341.3
Predecessor | DTA and PBGC
Reportable segment results
Net gain on disposal of assets				$ 19.7	19.7
Predecessor | Powder River Basin Mining | Operating Segments
Reportable segment results
Revenues					394.3
Total Adjusted EBITDA					91.7
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA					91.7
Predecessor | Midwestern U.S. Mining | Operating Segments
Reportable segment results
Revenues					193.2
Total Adjusted EBITDA					50.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA					50.0
Predecessor | Western U.S. Mining | Operating Segments
Reportable segment results
Revenues					149.7
Total Adjusted EBITDA					50.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA					50.0
Predecessor | Australian Metallurgical Mining | Operating Segments
Reportable segment results
Revenues					328.9
Total Adjusted EBITDA					109.6
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA					109.6
Predecessor | Australian Thermal Mining | Operating Segments
Reportable segment results
Revenues					224.8
Total Adjusted EBITDA					75.6
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA					75.6
Predecessor | Trading and Brokerage | Corporate, Non-Segment
Reportable segment results
Revenues					15.0
Total Adjusted EBITDA					8.8
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA					8.8
Predecessor | Corporate and Other | Corporate, Non-Segment
Reportable segment results
Revenues	[1]				20.3
Total Adjusted EBITDA					(44.4)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA					$ (44.4)

[1] Corporate and Other revenue includes unrealized gains and losses related to mark-to-market activity from financial contract hedge activities intended to secure pricing related to certain coal sales contracts. During the three and six months ended June 30, 2018, such net unrealized losses were $48.1 million and $9.5 million, respectively. During the periods April 2 through June 30, 2017 and January 1 through April 1, 2017, such net unrealized gains were $9.4 million and $16.6 million, respectively. When such gains and losses are realized in connection with recognition of the underlying transaction, they are reclassified to realized gains and losses and are then reflected in Trading and Brokerage revenue (realized losses of $9.6 million and $29.5 million during the three and six months ended June 30, 2018, respectively, and realized losses of $7.7 million and $11.1 million during the periods April 2 through June 30, 2017 and January 1 through April 1, 2017, respectively). At June 30, 2018 and December 31, 2017, the financial contracts’ fair values resulted in net liabilities of $48.4 million and $38.9 million, respectively. Assets and liabilities related to the financial contracts were deemed Level 2 according to the U.S. GAAP fair value hierarchy as the valuations are corroborated by the use of market-based pricing.

[2] Includes the gain of $20.6 million on the sale of certain surplus land assets in Queensland and the gain of $7.1 million recognized on the sale of the Company’s interest in the RMJV during the six months ended June 30, 2018 and the gain of $19.7 million recognized on the sale of Dominion Terminal Associates during the period January 1 through April 1, 2017, as described in Note 15. “Other Events”.