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Coal Trading - Textuals (Details) - Coal Trading - USD ($) | 3 Months Ended | 6 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Apr. 01, 2017 | Sep. 30, 2017 | Sep. 30, 2018 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Potential collateralization that may be requested by counterparties related to material adverse event | $ 9,700,000 |  |  |  | $ 9,700,000 |  |  |  |  | $ 7,000,000
Collateral required to be posted to counterparties |  |  |  |  |  |  |  |  |  | 400,000
External Credit Rating, Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 93.00% |  |  |  |  |
External Credit Rating, Non Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 0.00% |  |  |  |  |
Non Rated [Member] | Credit Concentration Risk [Member] | Coal Trading Positions |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 7.00% |  |  |  |  |
Successor |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Threshold for Unobservable Inputs to the Total Fair Value of Derivative Contracts |  |  |  | 10.00% |  |  |  |  |  |
Transfers Between Hierarchy Levels | 0 | $ 0 |  | $ 0 | $ 0 |  |  |  |  |
Total asset (liabilities) from coal trading activities, net | 69,300,000 |  |  |  | 69,300,000 |  |  |  |  | 35,800,000
Initial margin posted | 19,400,000 |  |  |  | 19,400,000 |  |  |  |  | 18,800,000
Receivables from Brokers-Dealers and Clearing Organizations | 200,000 |  |  |  | 200,000 |  |  |  |  | $ 1,800,000
Predecessor |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Threshold for Unobservable Inputs to the Total Fair Value of Derivative Contracts |  |  | 10.00% |  |  |  |  |  |  |
Transfers Between Hierarchy Levels |  |  | $ 0 |  |  |  |  |  |  |
Level 3 | Successor |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3 |  | $ 0 |  |  |  |  |  |  |  |
Level 3 | Scenario, Forecast |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Estimated Future Realization of Trading Portfolio |  |  |  |  |  | $ 200,000 | $ 5,300,000 | $ 10,900,000 | $ (12,000,000) |
Level 3 | Predecessor |  |  |  |  |  |  |  |  |  |
Concentration Risk [Line Items] |  |  |  |  |  |  |  |  |  |
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Transfers out of Level 3 | $ 0 |  | $ 0 | $ 700,000 | $ 0 |  |  |  |  |

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