v2.4.0.8
Derivative Instruments (Details 3) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Power contracts included in operating revenues $ 158 $ 15 $ (79) $ (59)
Natural gas contracts included in fuel and purchased energy expense 1 19 126 64
Interest rate swaps included in interest expense 1 2 2 4
Gain (Loss) on Derivative Instruments, Net, Pretax 160 36 49 9
Gain (Loss) on Sale of Derivatives 18 [1] 5 [1] (21) [1] 33 [1]
Mark-to-market gain (loss) 142 [2] 31 [2] 70 [2] (24) [2]
Interest Rate Swap [Member]
        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Mark-to-market gain (loss) 1 [2] 2 [2] 2 [2] 4 [2]
Commodity Option [Member]
        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Gain (Loss) on Sale of Derivatives 18 [1] 5 [1] (21) [1] 33 [1]
Mark-to-market gain (loss) $ 141 [2] $ 29 [2] $ 68 [2] $ (28) [2]
[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.