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Risk Management and Hedging Activities - Not Designated as Hedging Contracts (Details) - Not Designated as Hedging Instrument [Member] - Commodity Contract [Member] - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]        
Beginning balance $ 99 $ 0 $ 85 $ 55
Changes in fair value recognized in regulatory assets 38 34 65 (15)
Net gains (losses) reclassified to operating revenue 1 2 29 (9)
Net losses reclassified to energy costs (10) (7) (51) (2)
Ending balance $ 128 $ 29 $ 128 $ 29