Available-For-Sale Securities (Value Of Securities That Have Been In An Unrealized Loss Position For Less Than And Greater Than 12 Months) (Detail) (Power [Member], Nuclear Decommissioning Trust (NDT) Fund [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Unrealized Loss Position Less Than 12 Months [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 212	$ 204
Gross Unrealized Losses	(6)	(6)
Unrealized Loss Position Less Than 12 Months [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	67	139
Gross Unrealized Losses	(4)	(6)
Unrealized Loss Position Less Than 12 Months [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	92	34
Gross Unrealized Losses	(1)	0
Unrealized Loss Position Less Than 12 Months [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	53	31
Gross Unrealized Losses	(1)	0
Unrealized Loss Position Less Than 12 Months [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	145	65
Gross Unrealized Losses	2	0
Unrealized Loss Position Less Than 12 Months [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Gross Unrealized Losses	0	0
Unrealized Loss Position Greater Than 12 Months [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	2	7
Gross Unrealized Losses		0
Unrealized Loss Position Greater Than 12 Months [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Gross Unrealized Losses	0	0
Unrealized Loss Position Greater Than 12 Months [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1	1
Gross Unrealized Losses	0	0
Unrealized Loss Position Greater Than 12 Months [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	1	6
Gross Unrealized Losses	0	0
Unrealized Loss Position Greater Than 12 Months [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	2	7
Gross Unrealized Losses	0	0
Unrealized Loss Position Greater Than 12 Months [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	0
Gross Unrealized Losses	0	0
Unrealized Loss Position Greater Than 12 Months [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	$ 0