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Available-For-Sale Securities (Proceeds From The Sales Of And The Net Realized Gains On Securities In The NDT Funds And Rabbi Trusts) (Detail) (Power [Member], Nuclear Decommissioning Trust (NDT) Fund [Member], USD $)
In Millions, unless otherwise specified
1 Months Ended 12 Months Ended
Sep. 30, 2012
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Power [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
       
Schedule of Available-for-sale Securities [Line Items]        
Proceeds from Sale and Maturity of Available-for-sale Securities   $ 1,433 $ 1,355 $ 958
Gross Realized Gains 59 153 144 119
Gross Realized Losses   (52) (45) (39)
Net Realized Gains (Losses)   $ 101 $ 99 $ 80