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Asset Impairment and Mine Closure Costs Textuals (Details) - Predecessor - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Nov. 30, 2016
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Impairment of Long-Lived Assets to be Disposed of (after tax) | $ 193.2 |  |  |  |  |
Impairment of Long-Lived Assets Held-for-use |  |  |  | $ 247.9 | $ 1,001.3 |
Equity Method Investment, Other than Temporary Impairment |  |  |  |  | 276.5 |
Midwestern U.S. Mining [Member] |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Impairment of Long-Lived Assets Held-for-use |  | $ 9.7 |  | 54.7 | 40.2 |
Equity Method Investment, Other than Temporary Impairment |  |  |  |  | 0.0 |
Australian Mining |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Impairment of Long-Lived Assets Held-for-use |  | 144.5 | $ 230.5 | 193.2 | 675.2 |
Impairment of Long-Lived Assets to be Disposed of |  |  |  | 17.2 |  |
Equity Method Investment, Other than Temporary Impairment |  |  |  |  | 0.0 |
Remediation Property for Sale, Abandonment or Disposal [Member] |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Impairment of Long-Lived Assets Held-for-use |  | 317.7 |  |  |  |
Midwestern Inactive Surface Mine [Member] |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Impairment of Long-Lived Assets Held-for-use |  |  |  | 30.5 |  |
Corporate and Other |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Impairment of Long-Lived Assets Held-for-use |  | 86.2 |  | 37.5 | 268.4 |
Impairment of Long-Lived Assets to be Disposed of |  |  |  | 182.2 |  |
Middlemount Mine [Member] |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Equity Method Investment, Other than Temporary Impairment |  |  |  | 46.6 |  |
Subordinated Borrowing, Name [Domain] |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Equity Method Investment, Other than Temporary Impairment |  |  |  | 229.9 | 229.9 |
Middlemount Mine [Member] | Financing Receivable [Member] |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Investments and Other Noncurrent Assets | $ 84.8 | $ 65.2 |  | $ 84.8 | $ 65.2 |
Metropolitan Collieries Pty Ltd [Member] |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |
Planned Divestiture, Agreed Upon Proceeds from Divestiture |  |  |  |  |  | $ 200.0

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- Definition This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 35 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=99375318&loc=d3e32787-111569 + Details
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- Definition The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 958 -SubTopic 225 -Section 45 -Paragraph 11 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84241808&loc=d3e92212-112881 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2420-110228 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84176398&loc=d3e2921-110230 + Details
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