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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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