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Revenues - Revenue by Product Type and Market (Details) - USD ($) $ in Millions		3 Months Ended									9 Months Ended	12 Months Ended
	Apr. 01, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Apr. 01, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Successor
Disaggregation of Revenue [Line Items]
Revenues		$ 1,397.1	$ 1,412.6	$ 1,309.4	$ 1,462.7	$ 1,517.1	$ 1,477.2	$ 1,258.3			$ 4,252.6	$ 5,581.8
Successor | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											1,178.7	1,424.8
Successor | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											592.3	801.0
Successor | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											440.7	592.0
Successor | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											1,221.0	1,553.0
Successor | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											772.5	1,099.2
Successor | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											47.4	111.8
Successor | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											1,178.7	1,424.8
Successor | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											592.3	801.0
Successor | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											440.7	592.0
Successor | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											1,221.0	1,553.0
Successor | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											772.5	1,099.2
Successor | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											47.4	111.8
Successor | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues											2,967.3	3,888.7
Successor | Thermal coal | Domestic
Disaggregation of Revenue [Line Items]
Revenues											2,258.7	2,920.3
Successor | Thermal coal | Export
Disaggregation of Revenue [Line Items]
Revenues											708.6	968.4
Successor | Thermal coal | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											1,174.3	1,424.8
Successor | Thermal coal | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											592.0	800.7
Successor | Thermal coal | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											429.0	565.2
Successor | Thermal coal | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Thermal coal | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											772.0	1,098.0
Successor | Thermal coal | Operating Segments | Domestic | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											1,174.3	1,424.8
Successor | Thermal coal | Operating Segments | Domestic | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											586.7	799.4
Successor | Thermal coal | Operating Segments | Domestic | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											409.8	543.1
Successor | Thermal coal | Operating Segments | Domestic | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Thermal coal | Operating Segments | Domestic | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											87.9	153.0
Successor | Thermal coal | Operating Segments | Export | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Thermal coal | Operating Segments | Export | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											5.3	1.3
Successor | Thermal coal | Operating Segments | Export | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											19.2	22.1
Successor | Thermal coal | Operating Segments | Export | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Thermal coal | Operating Segments | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											684.1	945.0
Successor | Thermal coal | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Thermal coal | Corporate, Non-Segment | Domestic | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Thermal coal | Corporate, Non-Segment | Export | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues											1,221.0	1,548.6
Successor | Metallurgical coal | Export
Disaggregation of Revenue [Line Items]
Revenues											1,221.0	1,548.6
Successor | Metallurgical coal | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											1,221.0	1,548.6
Successor | Metallurgical coal | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Operating Segments | Export | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Operating Segments | Export | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Operating Segments | Export | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Operating Segments | Export | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											1,221.0	1,548.6
Successor | Metallurgical coal | Operating Segments | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Metallurgical coal | Corporate, Non-Segment | Export | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Other
Disaggregation of Revenue [Line Items]
Revenues											64.3	144.5
Successor | Other | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											4.4	0.0
Successor | Other | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											0.3	0.3
Successor | Other | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											11.7	26.8
Successor | Other | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	4.4
Successor | Other | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											0.5	1.2
Successor | Other | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											$ 47.4	$ 111.8
Predecessor
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0								$ 1,326.2	$ 1,326.2			$ 4,715.3
Predecessor | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									394.3				1,473.3
Predecessor | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2				792.5
Predecessor | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									149.7				526.0
Predecessor | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									328.9				1,090.4
Predecessor | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									224.8				824.9
Predecessor | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									35.3				8.2
Predecessor | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									394.3				1,473.3
Predecessor | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2				792.5
Predecessor | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									149.7				526.0
Predecessor | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									328.9				1,090.4
Predecessor | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									224.8				824.9
Predecessor | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									35.3				8.2
Predecessor | Thermal coal
Disaggregation of Revenue [Line Items]
Revenues									945.5				3,605.4
Predecessor | Thermal coal | Domestic
Disaggregation of Revenue [Line Items]
Revenues									748.3				2,890.1
Predecessor | Thermal coal | Export
Disaggregation of Revenue [Line Items]
Revenues									197.2				715.3
Predecessor | Thermal coal | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									394.3				1,472.5
Predecessor | Thermal coal | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2				793.4
Predecessor | Thermal coal | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									133.5				516.1
Predecessor | Thermal coal | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Thermal coal | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									224.5				823.4
Predecessor | Thermal coal | Operating Segments | Domestic | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									394.3				1,472.5
Predecessor | Thermal coal | Operating Segments | Domestic | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2				793.4
Predecessor | Thermal coal | Operating Segments | Domestic | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									133.5				516.1
Predecessor | Thermal coal | Operating Segments | Domestic | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Thermal coal | Operating Segments | Domestic | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									27.3				108.1
Predecessor | Thermal coal | Operating Segments | Export | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Thermal coal | Operating Segments | Export | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Thermal coal | Operating Segments | Export | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Thermal coal | Operating Segments | Export | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Thermal coal | Operating Segments | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									197.2				715.3
Predecessor | Thermal coal | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Thermal coal | Corporate, Non-Segment | Domestic | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Thermal coal | Corporate, Non-Segment | Export | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal
Disaggregation of Revenue [Line Items]
Revenues									324.6				1,089.6
Predecessor | Metallurgical coal | Export
Disaggregation of Revenue [Line Items]
Revenues									324.6				1,089.6
Predecessor | Metallurgical coal | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									324.6				1,089.6
Predecessor | Metallurgical coal | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Operating Segments | Export | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Operating Segments | Export | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Operating Segments | Export | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Operating Segments | Export | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									324.6				1,089.6
Predecessor | Metallurgical coal | Operating Segments | Export | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Metallurgical coal | Corporate, Non-Segment | Export | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Other
Disaggregation of Revenue [Line Items]
Revenues									56.1				20.3
Predecessor | Other | Operating Segments | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.8
Predecessor | Other | Operating Segments | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				(0.9)
Predecessor | Other | Operating Segments | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									16.2				9.9
Predecessor | Other | Operating Segments | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									4.3				0.8
Predecessor | Other | Operating Segments | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									0.3				1.5
Predecessor | Other | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									$ 35.3				$ 8.2

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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