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, 2012	Dec. 31, 2011
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 626	$ 582
Gross Unrealized Gains	128	126
Gross Unrealized Losses	(5)	(23)
Fair Value	749	685
Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	274	343
Gross Unrealized Gains	14	16
Gross Unrealized Losses	0	0
Fair Value	288	359
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	311	268
Gross Unrealized Gains	22	15
Gross Unrealized Losses	0	(2)
Fair Value	333	281
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	585	611
Gross Unrealized Gains	36	31
Gross Unrealized Losses	0	(2)
Fair Value	621	640
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	131	24
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	131	24
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	1,342	1,217
Gross Unrealized Gains	164	157
Gross Unrealized Losses	(5)	(25)
Fair Value	$ 1,501	$ 1,349