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Bad Debt on Physical Coal (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2015
Bad Debt on Physical Coal [Abstract] |  |  |  |  |  |  |
Unusual or Infrequent Item, or Both, Loss, Gross | $ 47.0 | $ 0.0 | $ 0.0 | $ 0.0 | $ 47.0 | $ 0.0 | $ 0.0

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- Definition Bad Debt on Physical Coal [Abstract] + References No definition available. + Details
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- Definition Amount, before deduction of insurance proceeds, of loss recognized in the income statement for an event or transaction that is unusual in nature or infrequent in occurrence, or both. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=64828382&loc=d3e15138-107781 + Details
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