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Segment and Related Information - Financial Data for Products and Services (Details) (USD $)
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Revenue from External Customer [Line Items]        
Electric Revenue $ 4,230,000,000 $ 4,166,000,000 $ 8,116,000,000 $ 7,755,000,000
Financial data for Products and Services        
Electric Utilities' Revenues 4,246,000,000 4,181,000,000 8,143,000,000 7,785,000,000
Retail [Member]
       
Revenue from External Customer [Line Items]        
Electric Revenue 3,620,000,000 3,597,000,000 6,918,000,000 6,689,000,000
Wholesale [Member]
       
Revenue from External Customer [Line Items]        
Electric Revenue 454,000,000 415,000,000 886,000,000 764,000,000
Other [Member]
       
Revenue from External Customer [Line Items]        
Electric Revenue $ 156,000,000 $ 154,000,000 $ 312,000,000 $ 302,000,000