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Discontinued Operations - Patriot Related Matters (Details) - USD ($) $ in Millions |  | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jan. 25, 2017 | Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2007 | Oct. 31, 2007
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Estimated fund obligation |  |  |  |  |  | $ 40.0
Amount contributed to the Combined Benefit Fund |  | $ 0.5 | $ 0.6 |  |  |
CombinedBenefitFundLowerEstimate |  | 1.0 |  |  |  |
CombinedBenefitFundFutureEstimate |  | 2.0 |  |  |  |
Combined Benefit Fund Liability |  | 16.0 |  | $ 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 |  |  |  |
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 |  | 103.1 |  | 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 |  | $ 37.8 |  | $ 36.7 |  |

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