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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 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]        
Beginning balance $ 130 $ 27 $ 85 $ 55
Changes in fair value recognized in regulatory assets (21) (27) 27 (49)
Net gains (losses) reclassified to operating revenue 3 0 28 (11)
Net (losses) gains reclassified to energy costs (13) 0 (41) 5
Ending balance $ 99 $ 0 $ 99 $ 0