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Discontinued Operations - Patriot Related Matters (Details) - Spinoff - Patriot - USD ($) $ in Millions | Jan. 25, 2017 | Sep. 30, 2018 | Dec. 31, 2007
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Black Lung Occupational Disease Liability |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
Initially Determined Potential Exposure from Patriot Bankruptcy |  |  | $ 150.0
Potential Exposure from Patriot Bankruptcy |  | $ 135.9 |
UMWA 1974 Pension Plan (UMWA Plan) Litigation |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
Settlement of claim, amount awarded to other party | $ 75.0 |  |
Litigation settlement, liability |  | $ 35.3 |

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