v2.4.0.6
Derivative Instruments (Details 3) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Power contracts included in operating revenues $ 329 $ 18 $ 149 $ 9
Natural gas contracts included in fuel and purchased energy expense 3 39 127 61
Interest rate swaps included in interest expense 3 2 9 4
Gain (Loss) on interest rate derivatives, net 0 (3) (14) (149)
Gain (Loss) on Derivative Instruments, Net, Pretax 335 [1] 56 [1] 271 [1] (75) [1]
Gain (Loss) on Sale of Derivatives 113 21 168 (33)
Unrealized gain (loss) 222 [1] 35 [1] 103 [1] (42) [1]
Interest Rate Swap [Member]
        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Gain (Loss) on Sale of Derivatives 0 (44) (157) (150)
Unrealized gain (loss) 3 [1] 43 [1] 152 [1] 5 [1]
Commodity Option [Member]
        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Gain (Loss) on Sale of Derivatives 113 65 325 117
Unrealized gain (loss) $ 219 [1] $ (8) [1] $ (49) [1] $ (47) [1]
[1] In addition to changes in market value on derivatives not designated as hedges, changes in unrealized 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