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Coal Trading - Fair Value (Details) - Coal Trading - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2017 | Apr. 01, 2017 | Jun. 30, 2016 | Jun. 30, 2016 | Dec. 31, 2016 | Jun. 30, 2015
Coal Trading (Textuals) [Abstract] |  |  |  |  |  |
Fair value hierarchy transfers from Level 1 to Level 2 | $ 0.0 |  |  |  |  | $ 0.0
Fair value hierarchy transfers from Level 2 to Level 1 | 0.0 |  |  |  |  | $ 0.0
Successor | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |
Physical purchase/sale contracts | (0.6) |  |  |  |  |
Total net financial assets (liabilities) | (0.6) |  |  |  |  |
Successor | Level 1 | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |
Physical purchase/sale contracts | 0.0 |  |  |  |  |
Total net financial assets (liabilities) | 0.0 |  |  |  |  |
Successor | Level 2 | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |
Physical purchase/sale contracts | (0.6) |  |  |  |  |
Total net financial assets (liabilities) | (0.6) |  |  |  |  |
Successor | Level 3 |  |  |  |  |  |
Changes in the Company's recurring Level 3 net financial assets |  |  |  |  |  |
Beginning of period | (0.7) |  |  |  |  |
Transfers into Level 3 | 0.0 |  |  |  |  |
Transfers out of Level 3 | 0.7 |  |  |  |  |
Included in earnings | 0.0 |  |  |  |  |
Sales | 0.0 |  |  |  |  |
Settlements | 0.0 |  |  |  |  |
End of period | 0.0 | $ (0.7) |  |  |  |
Changes in unrealized gains (losses) | 0.0 |  |  |  |  |
Successor | Level 3 | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |
Physical purchase/sale contracts | 0.0 |  |  |  |  |
Total net financial assets (liabilities) | 0.0 |  |  |  |  |
Predecessor | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |
Futures, swaps and options |  |  |  |  | $ (0.1) |
Physical purchase/sale contracts |  |  |  |  | (0.4) |
Total net financial assets (liabilities) |  |  |  |  | (0.5) |
Predecessor | Level 1 | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |
Futures, swaps and options |  |  |  |  | 0.0 |
Physical purchase/sale contracts |  |  |  |  | 0.0 |
Total net financial assets (liabilities) |  |  |  |  | 0.0 |
Predecessor | Level 2 | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |
Futures, swaps and options |  |  |  |  | (0.1) |
Physical purchase/sale contracts |  |  |  |  | 0.7 |
Total net financial assets (liabilities) |  |  |  |  | 0.6 |
Predecessor | Level 3 |  |  |  |  |  |
Changes in the Company's recurring Level 3 net financial assets |  |  |  |  |  |
Beginning of period | $ (0.7) | (1.1) | $ (3.9) | $ (15.6) |  |
Transfers into Level 3 |  | 0.0 | 0.4 | 0.4 |  |
Transfers out of Level 3 |  | 0.2 | 0.0 | 10.7 |  |
Included in earnings |  | 0.2 | (1.3) | (1.4) |  |
Sales |  | 0.0 | 0.0 | (0.1) |  |
Settlements |  | 0.0 | 3.7 | 4.9 |  |
End of period |  | (0.7) | (1.1) | (1.1) |  |
Changes in unrealized gains (losses) |  | $ 0.3 | $ (0.1) | $ (0.2) |  |
Predecessor | Level 3 | Fair Value, Measurements, Recurring |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |
Futures, swaps and options |  |  |  |  | 0.0 |
Physical purchase/sale contracts |  |  |  |  | (1.1) |
Total net financial assets (liabilities) |  |  |  |  | $ (1.1) |

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