Segment Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Successor
Reportable segment results
Revenues		$ 1,462.7
Total Adjusted EBITDA		363.9
Net gain on disposal of assets		30.6
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Income from continuing operations, net of income taxes		208.3
Depreciation, depletion and amortization		169.6
Asset retirement obligation expenses		12.3
Asset impairment		0.0
Changes in deferred tax asset valuation allowance and amortization of basis difference related to equity affiliates		(7.6)
Interest expense		36.3
Interest income		(7.2)
Reorganization items, net		(12.8)
Unrealized gains on economic hedges		(38.6)
Unrealized losses on non-coal trading derivative contracts		1.8
Take-or-pay contract-based intangible recognition		(8.3)
Income tax provision		10.1
Total Adjusted EBITDA		363.9
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		389.3
Total Adjusted EBITDA		74.5
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		74.5
Successor | Midwestern U.S. Mining | Operating Segments
Reportable segment results
Revenues		201.7
Total Adjusted EBITDA		31.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		31.2
Successor | Western U.S. Mining | Operating Segments
Reportable segment results
Revenues		143.7
Total Adjusted EBITDA		32.0
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		32.0
Successor | Australian Metallurgical Mining | Operating Segments
Reportable segment results
Revenues		466.2
Total Adjusted EBITDA		166.4
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		166.4
Successor | Australian Thermal Mining | Operating Segments
Reportable segment results
Revenues		201.4
Total Adjusted EBITDA		61.6
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		61.6
Successor | Trading and Brokerage | Corporate, Non-Segment
Reportable segment results
Revenues		20.1
Total Adjusted EBITDA		1.2
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA		1.2
Successor | Corporate and Other | Corporate, Non-Segment
Reportable segment results
Revenues		40.3
Total Adjusted EBITDA	[1]	(3.0)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA	[1]	$ (3.0)
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 from continuing operations, net of income taxes			124.3
Depreciation, depletion and amortization			119.9
Asset retirement obligation expenses			14.6
Asset impairment			30.5
Changes in deferred tax asset valuation allowance and amortization of basis difference related to equity affiliates			(5.2)
Interest expense			32.9
Interest income			(2.7)
Reorganization items, net			41.4
Unrealized gains on economic hedges			(16.6)
Unrealized losses on non-coal trading derivative contracts			0.0
Take-or-pay contract-based intangible recognition			0.0
Income tax provision			2.2
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			20.3
Total Adjusted EBITDA	[1]		(44.4)
Reconciliation Of Adjusted EBITDA To Consolidated Loss From Continuing Operations, Net of Income Taxes [Abstract]
Total Adjusted EBITDA	[1]		$ (44.4)

[1] 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 three months ended March 31, 2018 and the gain of $19.7 million recognized on the sale of Dominion Terminal Associates during the three months ended March 31, 2017, as described in Note 15. “Other Events”.