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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.