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Discontinued Operations (Details) AUD in Millions, $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2016 AUD | Sep. 30, 2016 AUD
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Disposal Group, Including Discontinued Operation, Other Assets, Current | $ 0.2 |  |  |  | $ 0.2 | $ 0.2 | $ 3.1 |  |  |
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax |  |  |  |  |  | (57.6) | (182.2) | $ (23.8) |  |
Discontinued Operation, Tax Effect of Discontinued Operation |  |  |  |  |  | 0.0 | (7.2) | 4.4 |  |
Loss from discontinued operations, net of income taxes | 13.1 | $ 38.1 | $ 155.1 | $ 7.6 | (53.6) | (57.6) | (175.0) | $ (28.2) |  |
Disposal Group, Including Discontinued Operation, Assets, Noncurrent | 15.9 |  |  |  | 15.9 | 15.9 | 13.2 |  |  |
Disposal Group, Including Discontinued Operation, Assets | 16.1 |  |  |  | 16.1 | 16.1 | 16.3 |  |  |
Disposal Group, Including Discontinued Operation, Liabilities, Current | 55.9 |  |  |  | 55.9 | 55.9 | 60.0 |  |  |
Disposal Group, Including Discontinued Operation, Other Liabilities, Noncurrent | 198.5 |  |  |  | 198.5 | 198.5 | 203.7 |  |  |
Liabilities Subject to Compromise | 8,440.2 |  |  |  | 8,440.2 | 8,440.2 | 0.0 |  |  |
Disposal Group, Including Discontinued Operation, Liabilities | 275.3 |  |  |  | 275.3 | 275.3 | 263.7 |  |  |
United Mine Workers of America [Member] |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Loss from discontinued operations, net of income taxes | 54.3 |  |  |  |  |  |  |  |  |
Wilkie Creek [Member] |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Loss Contingency, Loss in Period |  |  |  |  |  |  | 9.7 |  |  |
Settlement charges total | 9.9 |  |  |  | 9.9 | 9.9 |  |  | AUD 13.0 | AUD 13.0
Discontinued Operations [Member] |  |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |  |
Liabilities Subject to Compromise | $ 20.9 |  |  |  | $ 20.9 | $ 20.9 | $ 0.0 |  |  |

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