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	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,375	$ 1,366
Gross Unrealized Gains	233	180
Gross Unrealized Losses	(6)	(6)
Fair Value	1,602	1,540
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	628	648
Gross Unrealized Gains	205	147
Gross Unrealized Losses	(4)	(6)
Fair Value	829	789
Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	370	274
Gross Unrealized Gains	9	11
Gross Unrealized Losses	(1)	0
Fair Value	378	285
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	309	320
Gross Unrealized Gains	19	22
Gross Unrealized Losses	(1)	0
Fair Value	327	342
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	679	594
Gross Unrealized Gains	28	33
Gross Unrealized Losses	(2)	0
Fair Value	705	627
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	68	124
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 68	$ 124