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Risk Management and Hedging Activities - Not Designated as Hedging Contracts (Details) - Not Designated as Hedging Instrument [Member] - Commodity derivative [Member] - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]    
Beginning balance $ 250 $ 223
Changes in fair value recognized in net regulatory assets 36 60
Net gains reclassified to operating revenue 0 9
Net losses reclassified to cost of sales (33) (37)
Ending balance $ 253 $ 255