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Coal Trading Revenues by Type of Instrument (Details) - Coal Trading [Member] - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Apr. 01, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Predecessor |  |  |  |
Principal Transaction Revenue [Line Items] |  |  |  |
Trading Gain (Loss) | $ 15.0 |  | $ 28.9 | $ 40.6
Predecessor | Commodity Swaps and Options [Member] |  |  |  |
Principal Transaction Revenue [Line Items] |  |  |  |
Trading Gain (Loss) | (10.2) |  | (66.5) | 107.3
Predecessor | Physical commodity purchase / sale contracts [Member] |  |  |  |
Principal Transaction Revenue [Line Items] |  |  |  |
Trading Gain (Loss) | $ 25.2 |  | $ 95.4 | $ (66.7)
Successor |  |  |  |
Principal Transaction Revenue [Line Items] |  |  |  |
Trading Gain (Loss) |  | $ 33.6 |  |
Successor | Commodity Swaps and Options [Member] |  |  |  |
Principal Transaction Revenue [Line Items] |  |  |  |
Trading Gain (Loss) |  | (37.7) |  |
Successor | Physical commodity purchase / sale contracts [Member] |  |  |  |
Principal Transaction Revenue [Line Items] |  |  |  |
Trading Gain (Loss) |  | $ 71.3 |  |

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