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Coal Trading - Textuals (Details) - Coal Trading - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	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	$ 9.7				$ 7.0
Collateral required to be posted to counterparties					0.0
External Credit Rating, Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	27.00%
External Credit Rating, Non Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	72.00%
Non Rated [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member]
Concentration Risk [Line Items]
Concentration Risk, Percentage	1.00%
Successor
Concentration Risk [Line Items]
Total asset (liabilities) from coal trading activities, net	$ 47.5				35.8
Initial margin posted	19.8				18.8
Receivables from Brokers-Dealers and Clearing Organizations	1.1				$ 1.8
Additional potential collateral requirements for a credit downgrade	0.0
Margin posted to counterparties related to credit rating	$ 0.0
Level 3 | Scenario, Forecast
Concentration Risk [Line Items]
Estimated Future Realization of Trading Portfolio		$ 2.2	$ 0.5	$ (4.9)

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