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Coal Trading (Details 2) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2016
Dec. 31, 2015
Concentration Risk [Line Items]    
Number of major credit rating agencies that adjusted corporate credit rating 3  
Coal Trading [Member]    
Concentration Risk [Line Items]    
Percentage Of Trading Portfolio Expiration Year One 19.00%  
Percentage Of Trading Portfolio Expiration Year Two 83.00%  
Percentage Of Trading Portfolio Expiration Year Three (2.00%)  
Percentage Of Trading Portfolio Expiration 100.00%  
Potential collateralization that may be requested by counterparties related to material adverse event $ 5.0 $ 21.0
Margin posted to counterparties related to material adverse event 6.0 0.0
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] 32.8 (10.7)
Initial margin posted 13.4 9.2
Margin in excess of the exchange-required variation and initial margin $ 4.0 (0.7)
External Credit Rating, Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Concentration Risk, Percentage 57.00%  
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 20.00%  
Non Rated [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Concentration Risk, Percentage 23.00%  
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]    
Quality Adjustment Level 3 Unobservable Input as Percentage of Overall Valuation 3.00%  
Location Differentials Level 3 Unobservable Input As Percentage of Overall Valuation 11.00%  
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation 27.00%  
Weighted Average [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Quality Adjustment Level 3 Unobservable Input as Percentage of Overall Valuation 2.00%  
Location Differentials Level 3 Unobservable Input As Percentage of Overall Valuation 11.00%  
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation 27.00%  
Minimum [Member] | Coal Trading [Member]    
Concentration Risk [Line Items]    
Quality Adjustment Level 3 Unobservable Input as Percentage of Overall Valuation 0.00%  
Location Differentials Level 3 Unobservable Input As Percentage of Overall Valuation 11.00%  
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation 27.00%  
[1] None of the net variation margin held at June 30, 2016 and December 31, 2015, respectively, related to cash flow hedges.