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Derivative Instruments (Details 3) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Power contracts included in operating revenues $ 1,442 $ 1,939 $ 3,088 $ 3,904
Natural gas contracts included in fuel and purchased energy expense 766 1,134 1,776 2,491
Interest expense 158 169 312 335
Gain (Loss) on Derivative Instruments, Net, Pretax 103 160 233 49
Gain (Loss) on Sale of Derivatives [1] 104 18 163 (21)
Mark-to-market gain (loss) [2] (1) 142 70 70
Power [Member]        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Power contracts included in operating revenues 115 158 234 (79)
Interest Rate Swap [Member]        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Interest expense 0 1 1 2
Mark-to-market gain (loss) [2] 0 1 1 2
Energy Related Derivative [Member]        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Gain (Loss) on Sale of Derivatives [1] 104 18 163 (21)
Mark-to-market gain (loss) [2] (1) 141 69 68
Natural Gas [Member]        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Natural gas contracts included in fuel and purchased energy expense $ (12) $ 1 $ (2) $ 126
[1] Does not include the realized value associated with derivative instruments that settle through physical delivery.
[2] In addition to changes in market value on derivatives not designated as hedges, changes in mark-to-market gain (loss) also includes de-designation of interest rate swap cash flow hedges and related reclassification from AOCI into earnings, hedge ineffectiveness and adjustments to reflect changes in credit default risk exposure.