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	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,366	$ 1,217
Gross Unrealized Gains	180	157
Gross Unrealized Losses	(6)	(25)
Fair Value	1,540	1,349
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	648	582
Gross Unrealized Gains	147	126
Gross Unrealized Losses	(6)	(23)
Fair Value	789	685
Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	274	343
Gross Unrealized Gains	11	16
Gross Unrealized Losses	0	0
Fair Value	285	359
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	320	268
Gross Unrealized Gains	22	15
Gross Unrealized Losses	0	(2)
Fair Value	342	281
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	594	611
Gross Unrealized Gains	33	31
Gross Unrealized Losses		(2)
Fair Value	627	640
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	124	24
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 124	$ 24