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Discontinued Operations - Patriot Related Matters (Details) $ in Millions |  | 3 Months Ended | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jan. 25, 2017 USD ($) | Sep. 30, 2019 USD ($) retirees | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) retirees | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) | Oct. 09, 2015 buyers | Dec. 31, 2007 USD ($) | Oct. 31, 2007 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Number of Retirees | retirees |  | 0 |  | 0 |  |  |  |  |
Estimated fund obligation |  |  |  |  |  |  |  |  | $ 40.0
Amount contributed to the Combined Benefit Fund |  |  |  | $ 1.4 | $ 1.7 |  |  |  |
CombinedBenefitFundLowerEstimate |  | $ 1.0 |  |  |  |  |  |  |
CombinedBenefitFundFutureEstimate |  | 2.0 |  |  |  |  |  |  |
Combined Benefit Fund Liability |  | 15.4 |  | 15.4 |  | $ 16.4 |  |  |
Combined benefit fund [Member] |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax |  | 0.2 | $ 0.2 | 0.5 | $ 0.5 |  |  |  |
Patriot |  |  |  |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |  |  |  |
Number of Buyers | buyers |  |  |  |  |  |  | 2 |  |
Spinoff | Patriot | 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 |  | 103.8 |  | 103.8 |  | 102.7 |  |  |
Spinoff | UMWA 1974 Pension Plan (UMWA Plan) Litigation | Patriot |  |  |  |  |  |  |  |  |
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 |  | $ 24.9 |  | $ 24.9 |  | $ 36.7 |  |  |

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