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Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) (Details) | 3 Months Ended | 9 Months Ended | 12 Months Ended | 48 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2016 USD ($) | Sep. 30, 2016 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2017 USD ($) | Jan. 31, 2014 USD ($) | Oct. 31, 2007 USD ($)
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
CreditSupportToPatriotNetLettersOfCredit | $ 20,900,000 |  |  |  | $ 20,900,000 |  |  | $ 20,900,000 |  |  |  |  |
Letters of credit in support of reclamation obligations or activities | 80,000,000 | $ 15,700,000 |  |  | 80,000,000 | $ 15,700,000 |  | 80,000,000 | $ 126,600,000 |  |  |  |
Charge to Patriot credit support |  |  |  |  |  |  |  |  | 34,700,000 |  |  |  |
Correction of error in Patriot credit support liability |  |  |  |  |  |  |  |  | 16,600,000 |  |  |  |
Potential exposure from Patriot bankruptcy | $ 150,000,000 |  |  |  | $ 150,000,000 |  |  | $ 150,000,000 |  |  |  |  |
Liability recorded related to Patriot Bankruptcy |  |  |  |  |  |  |  | 123.3 |  |  |  |  |
Portion of Patriot Liability Paid |  |  |  |  |  |  |  | 0.7 |  |  |  |  |
estimated fund onbligation |  |  |  |  |  |  |  |  |  |  |  |  | $ 40,000,000
Patriot charge to loss from discontinued operations |  |  |  |  |  |  |  | $ (57,600,000) | (182,200,000) | $ (23,800,000) |  |  |
Amount paid to the Combined Benefit Fund |  |  |  |  |  |  |  | 2.6 |  |  |  |  |
Combined Benefit Fund Lower Estimate |  |  |  |  |  |  |  | $ 2,000,000 |  |  |  |  |
Combined Benefit Fund Future Estimate |  |  |  |  |  |  |  | $ 3,000,000 |  |  |  |  |
Liability related to Combined Benefit Fund |  |  |  |  |  |  |  | 22.7 |  |  |  |  |
Number of VEBA payments from Patriot settlement | 4 |  |  |  | 4 |  |  | 4 |  |  |  |  |
Initial Payment based on the negotiated settlement |  |  |  |  |  |  |  |  |  |  |  | $ 90,000,000 |
Payment to Patriot based on the construct of the negotiated settlement |  |  |  |  |  |  |  |  |  |  |  | $ 70,000,000 |
Payment to the VEBA based on the construct of the negotiated settlement |  |  |  |  |  |  |  | $ 75,000,000 | 75,000,000 | 20,000,000 |  |  |
Gain on VEBA settlement |  |  |  |  |  | 68,100,000 |  | $ 68,100,000 | 0 | 0 |  |  |
Number of annual payments to VEBA | 5 |  |  |  | 5 |  |  | 5 |  |  |  |  |
Payment to VEBA for start up and administrative costs |  |  |  |  |  |  |  | $ 100,000 |  |  |  |  |
Withdrawal liability related to pension plan |  | 767,000,000 |  |  |  | $ 767,000,000 |  |  |  |  |  |  |
Loss from discontinued operations, net of income taxes | $ 13,100,000 | $ 38,100,000 | $ 155,100,000 | $ 7,600,000 | $ (53,600,000) |  |  | (57,600,000) | (175,000,000) | $ (28,200,000) |  |  |
Scenario, Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Funding of the newly established VEBA |  |  |  |  |  |  |  |  |  |  | $ 310,000,000 |  |
Payment to the VEBA based on the construct of the negotiated settlement |  |  |  |  |  |  | $ 70,000,000 |  |  |  |  |  |
Combined benefit fund [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Patriot charge to loss from discontinued operations |  |  |  |  |  |  |  | 1,200,000 | 24,600,000 |  |  |  |
Retiree Health Care Obligations [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payment to the VEBA based on the construct of the negotiated settlement |  |  |  |  |  |  |  | $ 16,100,000 |  |  |  |  |
United Mine Workers of America [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Loss from discontinued operations, net of income taxes | $ 54,300,000 |  |  |  |  |  |  |  |  |  |  |  |
Veba Payment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payment to the VEBA based on the construct of the negotiated settlement |  |  |  |  |  |  |  |  | 7,500,000 |  |  |  |
Plan Effective Date [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payment to the VEBA based on the construct of the negotiated settlement |  |  |  |  |  |  |  |  | 5,000,000 |  |  |  |
90 Days after Plan Effective Date [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payment to the VEBA based on the construct of the negotiated settlement |  |  |  |  |  |  |  |  | 10,000,000 |  |  |  |
One year after previous payment date [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payment to the VEBA based on the construct of the negotiated settlement |  |  |  |  |  |  |  |  | 15,000,000 |  |  |  |
Remaining 3 years [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Matters Related to the Bankruptcy of Patriot Coal Corporation (Patriot) [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payment to the VEBA based on the construct of the negotiated settlement |  |  |  |  |  |  |  |  | $ 15,000,000 |  |  |  |

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