v2.4.0.6
Derivative Instruments (Details 3) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Derivative Instruments, Gain (Loss) [Line Items]      
Realized gain (loss) on derivatives $ (50) $ 83 $ 5
Unrealized gain (loss) 30 [1] (56) [1] 87 [1]
Summary of Derivative Instruments by Risk Exposure [Abstract]      
Power contracts included in operating revenues (20) (19) 7
Natural gas contracts included in fuel and purchased energy expense 138 276 109
Interest rate swaps included in interest expense 7 (7) (24)
Gain (Loss) on interest rate derivatives, net 145 223 0
Gain (Loss) on Derivative Instruments, Net, Pretax (20) [1] 27 [1] 92 [1]
Interest Rate Swap [Member]
      
Derivative Instruments, Gain (Loss) [Line Items]      
Realized gain (loss) on derivatives (193) (31) (32)
Unrealized gain (loss) 55 [1] (199) [1] 8 [1]
Commodity Option [Member]
      
Derivative Instruments, Gain (Loss) [Line Items]      
Realized gain (loss) on derivatives 143 114 37
Unrealized gain (loss) $ (25) [1] $ 143 [1] $ 79 [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 income, hedge ineffectiveness and adjustments to reflect changes in credit default risk exposure.