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	Jun. 30, 2012	Dec. 31, 2011
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 583	$ 582
Gross Unrealized Gains	155	126
Gross Unrealized Losses	(10)	(23)
Fair Value	728	685
Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	291	343
Gross Unrealized Gains	15	16
Gross Unrealized Losses	0	0
Fair Value	306	359
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	303	268
Gross Unrealized Gains	17	15
Gross Unrealized Losses	0	(2)
Fair Value	320	281
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	594	611
Gross Unrealized Gains	32	31
Gross Unrealized Losses	0	(2)
Fair Value	626	640
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	63	24
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	63	24
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	1,240	1,217
Gross Unrealized Gains	187	157
Gross Unrealized Losses	(10)	(25)
Fair Value	$ 1,417	$ 1,349