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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, 2017
Mar. 31, 2016
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]    
Beginning balance $ 148 $ 250
Changes in fair value recognized in net regulatory assets 33 36
Net gains reclassified to operating revenue 13 0
Net losses reclassified to cost of sales (14) (33)
Ending balance $ 180 $ 253