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Risk Management and Hedging Activities - Not Designated as Hedging Contracts (Details) - Commodity Contract [Member] - Not Designated as Hedging Instrument [Member] - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Regulatory Assets (Liabilities), Net, Derivatives [Roll Forward]      
Beginning balance $ 250 $ 223 $ 182
Changes In Fair Value Derivatives Recognized In Regulatory Assets Or Liabilities, Net (30) 128 96
Net Gains (Losses) Reclassified To Operating Revenue (5) 1 (32)
Net losses reclassified to cost of sales (67) (102) (23)
Ending balance $ 148 $ 250 $ 223