[Line Items] for Fair Value of Assets and Liabilities from Coal Trading Activities [Table]

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Gains or losses (realized/unrealized).

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of purchases, (sales), issuances and (settlements) of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of purchases of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258

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Amount of sales of financial instrument classified as a derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Derivatives and Fair Value Measurements Changes in Recurring Level 3 Net Financial Assets (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2016
Successor
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items]
Beginning of period			$ 287.9
Total gains realized/unrealized:
End of period		$ 287.9
Successor \| Coal Trading
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items]
Beginning of period			0.0
Transfers into Level 3		0.0	0.0
Transfers out of Level 3		0.7	0.0
Total gains realized/unrealized:
Included in earnings		0.0	(1.7)
Purchases		0.0	10.0
Sales		0.0	0.0
Settlements		0.0	1.7
End of period		0.0	$ 10.0
Predecessor \| Coal Trading
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items]
Beginning of period	$ (1.1)	$ (0.7)		$ (15.6)
Transfers into Level 3	0.0			5.3
Transfers out of Level 3	0.2			(0.4)
Total gains realized/unrealized:
Included in earnings	0.2			(2.4)
Purchases	0.0			0.0
Sales	0.0			0.0
Settlements	0.0			12.0
End of period	$ (0.7)			$ (1.1)