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Discontinued Operations - Patriot Related Matters (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Oct. 09, 2015 buyer | Dec. 31, 2008 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |
Number of buyers | buyer |  |  |  | 2 |
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 | $ 87.2 | $ 90.1 |  |  | $ 150.0
Loss contingency accrual, mark-to-market adjustment | $ 2.1 | $ 4.2 | $ (18.3) |  |

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