Available-For-Sale Securities (Fair Values And Gross Unrealized Gains And Losses For The Securities Held In The NDT Fund) (Detail) (Power [Member], Nuclear Decommissioning Trust (NDT) Fund [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,401	$ 1,366
Gross Unrealized Gains	250	180
Gross Unrealized Losses	(16)	(6)
Fair Value	1,635	1,540
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	615	648
Gross Unrealized Gains	234	147
Gross Unrealized Losses	(6)	(6)
Fair Value	843	789
Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	411	274
Gross Unrealized Gains	5	11
Gross Unrealized Losses	(6)	0
Fair Value	410	285
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	313	320
Gross Unrealized Gains	11	22
Gross Unrealized Losses	(4)	0
Fair Value	320	342
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	724	594
Gross Unrealized Gains	16	33
Gross Unrealized Losses	(10)	0
Fair Value	730	627
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	62	124
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 62	$ 124