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Coal Trading - Fair Value (Details) - Coal Trading - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Apr. 01, 2017 | Sep. 30, 2017 | Sep. 30, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | Jun. 30, 2017 | Dec. 31, 2016
Successor | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Futures, swaps and options | $ (2.1) |  |  |  | $ (2.1) |  | $ (7.2) |  |
Physical purchase/sale contracts | (7.2) |  |  |  | (7.2) |  | (1.9) |  |
Total net financial assets (liabilities) | (9.3) |  |  |  | (9.3) |  | (9.1) |  |
Successor | Level 1 | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Futures, swaps and options | 0.0 |  |  |  | 0.0 |  | (3.0) |  |
Physical purchase/sale contracts | 0.0 |  |  |  | 0.0 |  | 0.0 |  |
Total net financial assets (liabilities) | 0.0 |  |  |  | 0.0 |  | (3.0) |  |
Successor | Level 2 | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Futures, swaps and options | (2.1) |  |  |  | (2.1) |  | (4.2) |  |
Physical purchase/sale contracts | (5.5) |  |  |  | (5.5) |  | (1.9) |  |
Total net financial assets (liabilities) | (7.6) |  |  |  | (7.6) |  | (6.1) |  |
Successor | Level 3 |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs | (1.7) | $ 0.0 | $ (0.7) | $ 0.0 | (1.7) | $ 0.0 | 0.0 | $ 0.0 | $ (1.1)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3 |  | 0.0 |  |  |  |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings | (1.7) | 0.0 |  |  | (1.7) |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) | (1.7) | $ 0.0 |  | 0.0 | (1.7) |  |  |  |
Successor | Level 3 | Fair Value, Measurements, Recurring |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Futures, swaps and options | 0.0 |  |  |  | 0.0 |  | 0.0 |  |
Physical purchase/sale contracts | (1.7) |  |  |  | (1.7) |  | 0.0 |  |
Total net financial assets (liabilities) | (1.7) |  |  |  | (1.7) |  | $ 0.0 |  |
Predecessor | Level 3 |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs |  |  | (0.7) |  |  |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3 | $ 0.0 |  | 0.0 | 0.7 | $ 0.0 |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Gain (Loss) Included in Earnings |  |  | 0.2 | $ 0.0 |  |  |  |  |
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) |  |  | $ 0.3 |  |  |  |  |  |
Minimum | Successor |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation | (35.60%) |  |  |  | (35.60%) |  |  |  |
Maximum | Successor |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation | (36.00%) |  |  |  | (36.00%) |  |  |  |
Weighted Average | Successor |  |  |  |  |  |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis: |  |  |  |  |  |  |  |  |
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation | (35.80%) |  |  |  | (35.80%) |  |  |  |

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