Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Fair Value of Derivative Contracts (Details)

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Summary of Significant Accounting Policies - Fair Value of Derivative Contracts (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Basis of Presentation    
Other comprehensive income (loss) $ 0 $ 0
Derivative Financial Instruments    
Fair value of commodity derivative contracts 3,100  
Morgan Stanley    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 707  
Barclays    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 600  
JP Morgan    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 586  
Citigroup    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 321  
Wells Fargo    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 273  
Scotiabank    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 215  
BNP Paribas    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 160  
Toronto Dominion Bank    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 77  
Canadian Imperial Bank of Commerce    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 39  
Fifth Third Bank    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 35  
Bank of Montreal    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 31  
SunTrust    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 19  
Capital One    
Derivative Financial Instruments    
Fair value of commodity derivative contracts 9  
Natixis    
Derivative Financial Instruments    
Fair value of commodity derivative contracts $ 1