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Discontinued Operations - Patriot Related Matters (Details) $ in Millions |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Jan. 25, 2017 USD ($) | Dec. 31, 2020 USD ($) retiree | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Oct. 09, 2015 buyer | Dec. 31, 2007 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Number of buyers | buyer |  |  |  |  | 2 |
Loss contingency accrual, mark-to-market adjustment |  |  | $ 0.1 |  |  |
Amount contributed to the combined benefit fund |  | $ 1.6 | 1.9 | $ 2.2 |  |
Combined benefit fund liability |  | 13.2 | 15.2 |  |  |
Minimum |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Combined benefit fund future estimate |  | 1.0 |  |  |  |
Maximum |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Combined benefit fund future estimate |  | 1.5 |  |  |  |
Combined benefit fund |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Loss contingency accrual, mark-to-market adjustment |  | $ (0.9) |  | (2.4) |  |
Number of retirees | retiree |  | 0 |  |  |  |
Estimated fund obligation |  | $ 40.0 |  |  |  |
Loss from discontinued operations before income taxes |  | 0.5 | 0.7 | 0.7 |  |
Patriot Coal Corporation | Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff | UMWA1974 Pension 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 |  |  |  |  |
Estimated litigation liability |  | 13.9 | 26.0 |  |  |
Patriot Coal Corporation | Black Lung Occupational Disease Liability | Disposal Group, Disposed of by Means Other than Sale, Not Discontinued Operations, Spinoff |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Loss contingency accrual |  | 90.1 | 85.7 |  |  | $ 150.0
Loss contingency accrual, mark-to-market adjustment |  | $ 4.2 | $ (18.3) | $ (33.7) |  |

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