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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, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]        
Beginning balance $ 162 $ 185 $ 148 $ 250
Changes in fair value recognized in net regulatory assets 10 18 43 5
Net (losses) gains reclassified to operating revenue (5) (3) 9 (6)
Net losses reclassified to cost of sales (5) (5) (38) (54)
Ending balance $ 162 $ 195 $ 162 $ 195