v2.4.0.8
Derivative Instruments (Details 3) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Power contracts included in operating revenues $ 2,187 $ 2,050 $ 6,091 $ 4,863
Natural gas contracts included in fuel and purchased energy expense 1,268 1,059 3,759 2,880
Interest expense 156 176 491 522
Gain (Loss) on Derivative Instruments, Net, Pretax 77 65 126 74
Gain (Loss) on Sale of Derivatives 59 [1] 27 [1] 38 [1] 60 [1]
Mark-to-market gain (loss) 18 [2] 38 [2] 88 [2] 14 [2]
Power [Member]
        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Power contracts included in operating revenues 53 18 (26) (41)
Interest Rate Swap [Member]
        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Interest expense 7 (5) 9 (1)
Mark-to-market gain (loss) 7 [2] (5) [2] 9 [2] (1) [2]
Energy Related Derivative [Member]
        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Gain (Loss) on Sale of Derivatives 59 [1] 27 [1] 38 [1] 60 [1]
Mark-to-market gain (loss) 11 [2] 43 [2] 79 [2] 15 [2]
Natural Gas [Member]
        
Summary of Derivative Instruments by Risk Exposure [Abstract]        
Natural gas contracts included in fuel and purchased energy expense $ 17 $ 52 $ 143 $ 116
[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.