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Coal Trading - Textuals (Details) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) agency
Concentration Risk [Line Items] |  |  |
Number of major credit rating agencies that adjusted corporate credit rating | agency |  |  | 1
Coal Trading |  |  |
Concentration Risk [Line Items] |  |  |
Potential collateralization that may be requested by counterparties related to material adverse event |  | $ 1.1 | $ 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] | Coal Trading |  |  |
Concentration Risk [Line Items] |  |  |
Concentration Risk, Percentage |  | 55.00% |
External Credit Rating, Non Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading |  |  |
Concentration Risk [Line Items] |  |  |
Concentration Risk, Percentage |  | 1.00% |
Non Rated [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading |  |  |
Concentration Risk [Line Items] |  |  |
Concentration Risk, Percentage |  | 44.00% |
Cash Flow Hedging | Coal Trading |  |  |
Concentration Risk [Line Items] |  |  |
Total asset (liabilities) from coal trading activities, net |  | $ 0.0 |
Successor | Coal Trading |  |  |
Concentration Risk [Line Items] |  |  |
Total asset (liabilities) from coal trading activities, net | [1] | 6.7 | 35.8
Initial margin posted |  | 18.5 | 18.8
Receivables from Brokers-Dealers and Clearing Organizations |  | 4.6 | $ 1.8
Additional potential collateral requirements for a credit downgrade |  | 0.0 |
Margin posted to counterparties related to credit rating |  | $ 0.0 |
[1] | None of the net variation margin posted at March 31, 2018 and December 31, 2017 related to cash flow hedges.

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