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Fair Value of Assets and Liabilities from Coal Trading Activities (Details) - Coal Trading [Member] - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Predecessor |  |
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset |  | $ 191.2
Derivative Liability, Fair Value, Gross Liability |  | 249.1
Derivative, Fair Value, Net |  | (57.9)
Derivative Asset, Fair Value, Gross Liability |  | (190.5)
Derivative Liability, Fair Value, Gross Asset |  | 190.5
Derivative Fair Value Gross Basis Reclassifications Net |  | 0.0
Derivative Asset, Fair Value, Amount Offset Against Collateral |  | 0.0
Derivative Liability, Fair Value, Amount Offset Against Collateral |  | (57.4)
Derivative, Fair Value, Amount Offset Against Collateral, Net |  | 57.4
Derivative Asset |  | 0.7
Derivative Liability |  | 1.2
Derivative Assets (Liabilities), at Fair Value, Net |  | $ (0.5)
Successor |  |
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items] |  |
Derivative Asset, Fair Value, Gross Asset | $ 77.1 |
Derivative Liability, Fair Value, Gross Liability | 122.0 |
Derivative, Fair Value, Net | (44.9) |
Derivative Asset, Fair Value, Gross Liability | (74.5) |
Derivative Liability, Fair Value, Gross Asset | 74.5 |
Derivative Fair Value Gross Basis Reclassifications Net | 0.0 |
Derivative Asset, Fair Value, Amount Offset Against Collateral | 0.0 |
Derivative Liability, Fair Value, Amount Offset Against Collateral | (35.8) |
Derivative, Fair Value, Amount Offset Against Collateral, Net | 35.8 |
Derivative Asset | 2.6 |
Derivative Liability | 11.7 |
Derivative Assets (Liabilities), at Fair Value, Net | (9.1) |
Cash Flow Hedging [Member] |  |
Fair Value of Assets and Liabilities from Coal Trading Activities [Line Items] |  |
Derivative, Fair Value, Amount Offset Against Collateral, Net | $ 0.0 |

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