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Summary Quarterly Financial Information Summary of Quarterly Financial Information Textuals (Details) - USD ($) $ in Millions |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Nov. 28, 2017 | Nov. 27, 2017 | Apr. 01, 2017 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Apr. 01, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Predecessor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mark-to-Market Adjustment on Pension and Postretirement Plans, Net |  |  |  |  |  |  | $ 0.0 |  |  |  |  |  |  | $ 0.0 | $ 0.0
Payments of Debt Restructuring Costs |  |  |  |  |  |  |  |  |  |  | $ 10.3 | $ 26.4 |  |  |
Asset impairment |  |  |  |  |  |  | 30.5 | $ 30.5 | $ 230.7 |  |  | 17.2 |  | 247.9 | 1,277.8
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  | 22.8 |  |  |  | 13.7 |  |  | 23.2 | 45.0
(Income) loss from equity affiliates |  |  |  |  |  |  | (15.0) | (15.0) | (28.8) |  |  |  |  | (16.2) | 15.9
Interest Expense |  |  |  |  |  |  | 32.9 |  |  |  |  | $ 126.2 |  | 298.6 | 465.4
Reorganization Items |  |  | $ 585.8 |  |  |  | 627.2 | $ 41.4 | 33.9 | $ 29.7 | 95.4 |  |  | 159.0 | 0.0
Loss on early debt extinguishment |  |  |  |  |  |  | 0.0 |  | 29.5 |  |  |  |  | (29.5) | (67.8)
Income (Loss) from discontinued operations, net of tax, including portion attributable to noncontrolling interest |  |  |  |  |  |  | (16.2) |  | $ 13.1 | $ 38.1 |  |  |  | (57.6) | (175.0)
Predecessor | Nonstrategic Australian mining tenement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  |  |  | 2.8 |  |  |  |
Predecessor | Prarie State Energy Campus [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  |  |  |  |  |  |  |  |  |  | $ 6.2 |  |  |  |
Predecessor | Burton Mine [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  | 0.0 |  |  |  |  |  |  | 0.0 | 0.0
Predecessor | Springfield Reclamation Liability [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  |  |  |  | $ 0.0 |  |  |  |  |  |  | $ 0.0 | $ 0.0
Successor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mark-to-Market Adjustment on Pension and Postretirement Plans, Net |  |  |  | $ 45.2 |  |  |  |  |  |  |  |  | $ (45.2) |  |
Payments of Debt Restructuring Costs |  |  |  | 6.6 |  |  |  |  |  |  |  |  |  |  |
Asset impairment |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 |  |
Gain (Loss) on Disposition of Assets |  |  |  | 83.1 |  |  |  |  |  |  |  |  | 84.0 |  |
(Income) loss from equity affiliates |  |  |  | (22.8) | $ (10.5) | $ (15.7) |  |  |  |  |  |  | (49.0) |  |
Interest Expense |  |  |  |  | 42.5 | $ 41.4 |  |  |  |  |  |  | 119.7 |  |
Reorganization Items |  |  |  |  |  |  |  |  |  |  |  |  | 0.0 |  |
Loss on early debt extinguishment |  |  |  | 8.0 | $ 12.9 |  |  |  |  |  |  |  | (20.9) |  |
Income (Loss) from discontinued operations, net of tax, including portion attributable to noncontrolling interest |  |  |  |  |  |  |  |  |  |  |  |  | (19.8) |  |
Successor | Burton Mine [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets |  | $ 52.2 |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  | 52.2 |  |  |  |  |  |  |  |  | (52.2) |  |
Successor | Springfield Reclamation Liability [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Quarterly Financial Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Assets | $ 31.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Sale of Properties |  |  |  | $ 31.2 |  |  |  |  |  |  |  |  | $ (31.2) |  |

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