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Available-For-Sale Securities (Proceeds From The Sales Of And The Net Realized Gains On Securities) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Proceeds from Sale and Maturity of Available-for-sale Securities $ 1,397 $ 1,448 $ 1,070
Gross Realized Gains 97 177 112
Gross Realized Losses (37) (23) (26)
Net Realized Gains (Losses) 60 154 86
Rabbi Trust [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Proceeds from Sale and Maturity of Available-for-sale Securities 104 467 89
Gross Realized Gains 3 4 4
Gross Realized Losses (2) (3) (3)
Net Realized Gains (Losses) $ 1 $ 1 $ 1