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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)

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- Definition Gains or losses (realized/unrealized). + References No definition available. + Details
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- Definition 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. + 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)(1) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition 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. + 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 + Details
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- Definition Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy. + 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)(3) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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