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Risk Management and Hedging Activities - Designated as Hedging Contracts (Details) - Designated as Hedging Instrument [Member] - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Accumulated Other Comprehensive (Income) Loss, Net, Derivatives [Roll Forward]    
Cash flow hedge ineffectiveness insignificant insignificant
Cash flow hedge loss to be reclassified within twelve months $ 51  
Commodity derivative [Member]    
Accumulated Other Comprehensive (Income) Loss, Net, Derivatives [Roll Forward]    
Beginning balance 46 $ 32
Changes in fair value recognized in OCI 48 (8)
Net gains reclassified to operating revenue 0 1
Net (losses) gains reclassified to cost of sales (22) 2
Ending balance $ 72 $ 27