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Fair Value Measurements and Credit Concentration (Net Asset and Liability Measured on Recurring Basis Level 3 Gain Loss Included in Earnings and Other Recurring Numeric Data) (Details) - Level 3 - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Net gains and losses on assets and liabilities [Abstract]    
Total gains (losses) included in earnings $ 60 $ (17)
Recurring | Energy Commodities, Net | Wholesale energy    
Net gains and losses on assets and liabilities [Abstract]    
Total gains (losses) included in earnings 41 21
Change in unrealized gains (losses) relating to positions still held at the reporting date 13 25
Recurring | Energy Commodities, Net | Retail Energy    
Net gains and losses on assets and liabilities [Abstract]    
Total gains (losses) included in earnings 18 (40)
Change in unrealized gains (losses) relating to positions still held at the reporting date 7 (9)
Recurring | Energy Commodities, Net | Fuel and energy purchases    
Net gains and losses on assets and liabilities [Abstract]    
Total gains (losses) included in earnings 1 2
Change in unrealized gains (losses) relating to positions still held at the reporting date $ (1) $ 1