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Discontinued Operations - Patriot Related Matters (Details) - Spinoff - Patriot - USD ($) $ in Millions |  | 6 Months Ended | 12 Months Ended | 24 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Jan. 25, 2017 | Jun. 30, 2016 | Dec. 31, 2013 | Dec. 31, 2015 | Jun. 30, 2017 | Dec. 31, 2007
Black Lung Occupational Disease Liability |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Potential exposure from Patriot bankruptcy |  |  |  |  | $ 124.6 | $ 150.0
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 | $ 54.3 |  |  |  | $ 52.6 |
Funding of VEBA Pursuant to Settlement with UMWA and Patriot Coal Corporation |  |  |  |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |  |  |  |
Funding of the newly established VEBA |  |  | $ 310.0 |  |  |
Number of years of payments to fund VEBA |  |  | 4 years |  |  |
Payments made to fund VEBA |  |  |  | $ 165.0 |  |
Settlement agreement, amount |  | $ 75.0 |  |  |  |
Gain on VEBA settlement |  | $ 68.1 |  |  |  |

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