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Derivative Instruments and Hedging Activities (Intro) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Cash Flow Hedges    
Net unrealized after-tax gains (losses) expected to be reclassified into earnings within 12 months related to commodity price risk cash flow hedge contracts $ 15  
Commodity Price Risk Nontrading Economic Activity Numeric [Abstract]    
Year of expiration of the maximum maturity date of economic activity derivative contracts 2020  
Wholesale energy    
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]    
Pre-tax gain (loss) $ 60 $ 19
Retail Energy    
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]    
Pre-tax gain (loss) 1 (13)
Fuel and energy purchases    
Pre-tax Gains (Losses) of Economic Activity (Details) [Abstract]    
Pre-tax gain (loss) $ 17 $ 34