Segment Information (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended	9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Apr. 01, 2017	Sep. 30, 2017	Sep. 30, 2018
Successor
Reportable segment results
Revenues		$ 1,412.6		$ 1,477.2		$ 2,735.5	$ 4,184.7
Total Adjusted EBITDA		372.1		411.3		729.1	1,105.6
Net gain on disposal of assets		20.8		0.4		0.9	49.8
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income (loss) from continuing operations, net of income taxes		83.9		233.7		335.1	412.2
Depreciation, depletion and amortization		169.6		194.5		342.8	503.1
Asset retirement obligation expenses		12.4		11.3		22.3	37.9
Asset impairment		0.0		0.0		0.0	0.0
Provision for North Goonyella equipment loss		49.3		0.0		0.0	49.3
Changes in deferred tax asset valuation allowance and amortization of basis difference related to equity affiliates		(6.1)		(3.4)		(7.7)	(22.1)
Interest expense		38.2		42.4		83.8	112.8
Loss on early debt extinguishment		0.0		12.9		12.9	2.0
Interest income		(10.1)		(2.0)		(3.5)	(24.3)
Reorganization items, net		0.0		0.0		0.0	(12.8)
Break Fees On Terminated Asset Sales		0.0		0.0		(28.0)	0.0
Unrealized losses (gains) on economic hedges		26.8		10.8		1.4	36.3
Unrealized (gains) losses on non-coal trading derivative contracts		(0.3)		1.7		(1.5)	1.4
Fresh-Start Adjustment, Revaluation of Inventory		0.0		0.0		67.3	0.0
Take-or-pay contract-based intangible recognition		(5.4)		(6.5)		(16.4)	(21.5)
Income tax provision (benefit)		13.8		(84.1)		(79.4)	31.3
Total Adjusted EBITDA		372.1		411.3		729.1	1,105.6
Successor | Wotonga Transaction
Reportable segment results
Net gain on disposal of assets							20.5
Successor | Iffley Land Transaction
Reportable segment results
Net gain on disposal of assets							20.6
Successor | Red Mountain Joint Venture
Reportable segment results
Net gain on disposal of assets							7.1
Successor | Powder River Basin Mining | Operating Segments
Reportable segment results
Revenues		373.7		420.9		786.3	1,084.5
Total Adjusted EBITDA		88.2		112.7		197.5	224.7
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		88.2		112.7		197.5	224.7
Successor | Midwestern U.S. Mining | Operating Segments
Reportable segment results
Revenues		208.5		207.7		402.6	607.7
Total Adjusted EBITDA		38.7		49.5		96.0	111.9
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		38.7		49.5		96.0	111.9
Successor | Western U.S. Mining | Operating Segments
Reportable segment results
Revenues		156.1		155.7		281.1	439.4
Total Adjusted EBITDA		28.5		34.5		79.4	94.4
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		28.5		34.5		79.4	94.4
Successor | Australian Metallurgical Mining | Operating Segments
Reportable segment results
Revenues		370.3		415.9		703.7	1,254.0
Total Adjusted EBITDA		90.7		143.1		215.0	415.6
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		90.7		143.1		215.0	415.6
Successor | Australian Thermal Mining | Operating Segments
Reportable segment results
Revenues		305.1		265.8		505.0	773.9
Total Adjusted EBITDA		145.3		97.8		203.7	314.5
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		145.3		97.8		203.7	314.5
Successor | Trading and Brokerage | Corporate, Non-Segment
Reportable segment results
Revenues		22.6		19.4		24.6	52.7
Total Adjusted EBITDA		(2.4)		2.7		(2.4)	1.9
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		(2.4)		2.7		(2.4)	1.9
Successor | Corporate and Other | Corporate, Non-Segment
Reportable segment results
Revenues	[1]	(23.7)		(8.2)		32.2	(27.5)
Total Adjusted EBITDA		(16.9)	[2]	(29.0)		(60.1)	(57.4)	[2]
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		$ (16.9)	[2]	$ (29.0)		$ (60.1)	$ (57.4)	[2]
Predecessor
Reportable segment results
Revenues					$ 1,326.2
Total Adjusted EBITDA					341.3
Net gain on disposal of assets					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					(195.5)
Depreciation, depletion and amortization					119.9
Asset retirement obligation expenses					14.6
Asset impairment					30.5
Provision for North Goonyella equipment loss					0.0
Changes in deferred tax asset valuation allowance and amortization of basis difference related to equity affiliates					(5.2)
Interest expense					32.9
Loss on early debt extinguishment					0.0
Interest income					(2.7)
Reorganization items, net					627.2
Break Fees On Terminated Asset Sales					0.0
Unrealized losses (gains) on economic hedges					(16.6)
Unrealized (gains) losses on non-coal trading derivative contracts					0.0
Fresh-Start Adjustment, Revaluation of Inventory					0.0
Take-or-pay contract-based intangible recognition					0.0
Income tax provision (benefit)					(263.8)
Total Adjusted EBITDA					341.3
Predecessor | DTA and PBGC
Reportable segment results
Net gain on disposal of assets					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	[2]				(44.4)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA	[2]				$ (44.4)

[1] Corporate and Other revenue includes unrealized gains and losses related to mark-to-market activity from economic hedge activities intended to hedge future coal sales. Such net unrealized losses were $26.8 million and $10.8 million during the three months ended September 30, 2018 and 2017, respectively, and $36.3 million and $1.4 million during the nine months ended September 30, 2018 and the period April 2 through September 30, 2017, respectively. During the period January 1 through April 1, 2017, such net unrealized gains were $16.6 million. 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 $11.6 million and $12.3 million during the three months ended September 30, 2018 and 2017, respectively, and $41.1 million, $20.0 million and $11.1 million during the nine months ended September 30, 2018 and the periods April 2 through September 30, 2017 and January 1 through April 1, 2017, respectively). At September 30, 2018 and December 31, 2017, the financial contracts’ fair values resulted in net liabilities, excluding margin, of $75.2 million and $38.9 million, respectively.

[2] ncluded in the three and nine months ended September 30, 2018 is the gain of $20.5 million recognized on the sale of surplus coal resources associated with the Millennium Mine. Also included in the nine months ended September 30, 2018 is the gain of $20.6 million recognized 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. Included in the period January 1 through April 1, 2017 is the gain of $19.7 million recognized on the sale of Dominion Terminal Associates