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, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,383	$ 1,366
Gross Unrealized Gains	218	180
Gross Unrealized Losses	(21)	(6)
Fair Value	1,580	1,540
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	636	648
Gross Unrealized Gains	202	147
Gross Unrealized Losses	(9)	(6)
Fair Value	829	789
Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	388	274
Gross Unrealized Gains	5	11
Gross Unrealized Losses	(8)	0
Fair Value	385	285
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	311	320
Gross Unrealized Gains	11	22
Gross Unrealized Losses	(4)	0
Fair Value	318	342
Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	699	594
Gross Unrealized Gains	16	33
Gross Unrealized Losses	(12)	0
Fair Value	703	627
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	48	124
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 48	$ 124