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Derivative Instruments and Hedging Activities (Fair Values) (Details) - USD ($)
$ in Millions
Mar. 31, 2016
Dec. 31, 2015
Fair Values by Balance Sheet Location [Abstract]    
Asset value $ 840 $ 693
Liability value 619 539
Not Designated as Hedging Instrument    
Fair Values by Balance Sheet Location [Abstract]    
Asset value 840 693
Liability value 619 539
Not Designated as Hedging Instrument | Current Assets | Energy Commodities, net    
Fair Values by Balance Sheet Location [Abstract]    
Asset value 680 562
Not Designated as Hedging Instrument | Noncurrent Assets | Energy Commodities, net    
Fair Values by Balance Sheet Location [Abstract]    
Asset value 160 131
Not Designated as Hedging Instrument | Current Liabilities | Energy Commodities, net    
Fair Values by Balance Sheet Location [Abstract]    
Liability value 492 431
Not Designated as Hedging Instrument | Noncurrent Liabilities | Energy Commodities, net    
Fair Values by Balance Sheet Location [Abstract]    
Liability value $ 127 $ 108