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Revenues - Revenue by Contract Duration (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 | One year or longer
Disaggregation of Revenue [Line Items]
Revenues											3,358.3	4,427.3
Successor | Less than one year
Disaggregation of Revenue [Line Items]
Revenues											830.0	1,010.0
Successor | Other
Disaggregation of Revenue [Line Items]
Revenues											64.3	144.5
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 | Operating Segments | One year or longer | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											1,023.1	1,283.9
Successor | Operating Segments | One year or longer | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											560.5	775.4
Successor | Operating Segments | One year or longer | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											404.6	531.8
Successor | Operating Segments | One year or longer | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											867.1	1,036.7
Successor | Operating Segments | One year or longer | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											503.0	799.5
Successor | Operating Segments | Less than one year | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											151.2	140.9
Successor | Operating Segments | Less than one year | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											31.5	25.3
Successor | Operating Segments | Less than one year | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											24.4	33.4
Successor | Operating Segments | Less than one year | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											353.9	511.9
Successor | Operating Segments | Less than one year | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											269.0	298.5
Successor | Operating Segments | Other | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues											4.4	0.0
Successor | Operating Segments | Other | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											0.3	0.3
Successor | Operating Segments | Other | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues											11.7	26.8
Successor | Operating Segments | Other | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues											0.0	4.4
Successor | Operating Segments | Other | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues											0.5	1.2
Successor | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											47.4	111.8
Successor | Corporate, Non-Segment | One year or longer | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Corporate, Non-Segment | Less than one year | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues											0.0	0.0
Successor | Corporate, Non-Segment | Other | 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 | One year or longer
Disaggregation of Revenue [Line Items]
Revenues									1,054.9				3,784.9
Predecessor | Less than one year
Disaggregation of Revenue [Line Items]
Revenues									215.2				910.1
Predecessor | Other
Disaggregation of Revenue [Line Items]
Revenues									56.1				20.3
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 | Operating Segments | One year or longer | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									357.7				1,364.0
Predecessor | Operating Segments | One year or longer | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									193.2				791.8
Predecessor | Operating Segments | One year or longer | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									129.3				478.8
Predecessor | Operating Segments | One year or longer | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									240.6				686.1
Predecessor | Operating Segments | One year or longer | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									134.1				464.2
Predecessor | Operating Segments | Less than one year | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									36.6				108.5
Predecessor | Operating Segments | Less than one year | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				1.6
Predecessor | Operating Segments | Less than one year | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									4.2				37.3
Predecessor | Operating Segments | Less than one year | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									84.0				403.5
Predecessor | Operating Segments | Less than one year | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									90.4				359.2
Predecessor | Operating Segments | Other | Powder River Basin Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.8
Predecessor | Operating Segments | Other | Midwestern U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									0.0				(0.9)
Predecessor | Operating Segments | Other | Western U.S. Mining
Disaggregation of Revenue [Line Items]
Revenues									16.2				9.9
Predecessor | Operating Segments | Other | Seaborne Metallurgical Mining
Disaggregation of Revenue [Line Items]
Revenues									4.3				0.8
Predecessor | Operating Segments | Other | Seaborne Thermal Mining
Disaggregation of Revenue [Line Items]
Revenues									0.3				1.5
Predecessor | Corporate, Non-Segment | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									35.3				8.2
Predecessor | Corporate, Non-Segment | One year or longer | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Corporate, Non-Segment | Less than one year | Corporate and Other
Disaggregation of Revenue [Line Items]
Revenues									0.0				0.0
Predecessor | Corporate, Non-Segment | Other | 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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