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Asset Impairment and Mine Closure Costs (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2016
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Impairment of Long-Lived Assets to be Disposed of (after tax) | $ 193.2 |  |  |  |  |  |  |  |
Long-lived assets |  |  |  |  |  | $ 247.9 | $ 1,001.3 | $ 149.7 |
Marketable securities |  |  |  |  |  | 0.0 | 4.7 | 4.7 |
Equity method investment |  |  |  |  |  |  | 276.5 |  |
Asset Impairment | 230.7 | $ 17.2 | $ 377.0 | $ 900.8 | $ 230.7 | 247.9 | 1,277.8 | 154.4 |
Asset realization risk | 1,407.3 |  |  |  | $ 1,407.3 | 1,407.3 |  |  |
Additional asset impairment charge deemed necessary for assets at risk |  |  |  |  |  | 0.0 |  |  |
Australian Metallurgical Mining [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Long-lived assets |  |  |  |  |  |  | 66.7 |  |
Australian Mining [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Impairment of long-lived assets related to mine closures |  |  |  |  |  | 17.2 |  |  |
Long-lived assets | 144.5 |  |  | $ 230.5 |  | 193.2 | 675.2 | 66.7 |
Marketable securities |  |  |  |  |  |  |  | 0.0 |
Equity method investment |  |  |  |  |  |  | 0.0 |  |
Asset Impairment |  |  |  |  |  |  | 675.2 | 66.7 |
Australian Thermal Mining [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Long-lived assets |  |  |  |  |  |  | 17.5 | 11.9 |
Marketable securities |  |  |  |  |  |  |  | 0.0 |
Equity method investment |  |  |  |  |  |  | 0.0 |  |
Asset Impairment |  |  |  |  |  |  | 17.5 | 11.9 |
Western U.S. Mining [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Long-lived assets |  |  |  |  |  |  |  | 2.7 |
Marketable securities |  |  |  |  |  |  |  | 0.0 |
Asset Impairment |  |  |  |  |  |  |  | 2.7 |
Midwestern U.S. Mining [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Long-lived assets | 9.7 |  |  |  |  | 37.5 | 40.2 |  |
Equity method investment |  |  |  |  |  |  | 0.0 |  |
Asset Impairment |  |  |  |  |  |  | 40.2 |  |
Corporate and Other [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Impairment of long-lived assets related to mine closures |  |  |  |  |  | 182.2 |  |  |
Long-lived assets | 86.2 |  |  |  |  | 54.7 | 268.4 | 68.4 |
Marketable securities |  |  |  |  |  |  |  | 4.7 |
Equity method investment |  |  |  |  |  |  | 276.5 |  |
Asset Impairment |  |  |  |  |  |  | 544.9 | $ 73.1 |
Remediation Property for Sale, Abandonment or Disposal [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Long-lived assets | $ 317.7 |  |  |  |  |  |  |  |
Midwestern Inactive Surface Mine [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Long-lived assets |  |  |  |  |  | $ 30.5 |  |  |
Middlemount Mine [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage | 50.00% |  |  |  | 50.00% | 50.00% |  |  |
Subordinated Borrowing, Name [Domain] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment |  |  |  |  |  | $ 229.9 | 229.9 |  |
Financing Receivable [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Investments and Other Noncurrent Assets | $ 84.8 |  | 65.2 |  | $ 84.8 | $ 84.8 | 65.2 |  |
Middlemount Mine [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage | 50.00% |  |  |  | 50.00% | 50.00% |  |  |
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 | $ 84.8 | $ 65.2 |  |
Middlemount Mine [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Equity method investment |  |  |  |  |  | $ 46.6 |  |  |
Metropolitan mine [Member] |  |  |  |  |  |  |  |  |
Impaired Long-Lived Assets Held and Used [Line Items] |  |  |  |  |  |  |  |  |
Cash consideration for asset held for sale |  |  |  |  |  |  |  |  | $ 200.0

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- Definition Additional asset impairment charges (in excess of those already taken, if any) deemed necessary for assets whose realization is most sensitive to near-term factors. + References No definition available. + Details
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- Definition Assets with values most sensitive to near-term coal pricing, including mines with comparatively shorter remaining lives and mining-related investments. + References No definition available. + Details
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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=49116238&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=51659069&loc=d3e92212-112881 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Impairment -URI http://asc.fasb.org/extlink&oid=6515133 Reference 3: 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=56944662&loc=d3e3602-108585 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=51719941&loc=d3e2921-110230 Reference 5: 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=51824106&loc=d3e2420-110228 + Details
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