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Coal Trading (Details 2)
$ in Millions
3 Months Ended
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Concentration Risk [Line Items]    
Number of major credit rating agencies that adjusted corporate credit rating 3  
Coal Trading [Member]    
Concentration Risk [Line Items]    
Potential collateralization that may be requested by counterparties related to material adverse event $ 1.0 $ 2.0
Margin posted to counterparties related to material adverse event   1.0
Additional potential collateral requirements for a credit downgrade 0.0 0.0
Margin posted to counterparties related to credit rating 0.0 0.0
Total asset (liabilities) from coal trading activities, net [1] 27.7 57.4
Initial margin posted 13.9 16.2
Margin in excess of the exchange-required variation and initial margin $ 0.2 2.0
External Credit Rating, Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Concentration Risk, Percentage 23.00%  
External Credit Rating, Non Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Concentration Risk, Percentage 47.00%  
Non Rated [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Concentration Risk, Percentage 30.00%  
Cash Flow Hedging [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Total asset (liabilities) from coal trading activities, net $ 0.0 $ 0.0
Risk Level, High [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation 26.00%  
Weighted Average [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation 26.00%  
Minimum [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation 26.00%  
[1] None of the net variation margin (held) posted at March 31, 2017 and December 31, 2016, respectively, related to cash flow hedges.