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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 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]        
Beginning balance $ 185 $ 233 $ 250 $ 223
Changes in fair value recognized in net regulatory assets 18 47 5 104
Net losses reclassified to operating revenue (3) (11) (6) (4)
Net losses reclassified to cost of sales (5) (16) (54) (70)
Ending balance $ 195 $ 253 $ 195 $ 253