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	Sep. 30, 2012		Dec. 31, 2011
Unrealized Loss Position Less Than 12 Months [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 215	[1]	$ 183	[1]
Gross Unrealized Losses	(5)	[1]	(23)	[1]
Unrealized Loss Position Less Than 12 Months [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	11	[2]	20	[2]
Gross Unrealized Losses	0	[2]	0	[2]
Unrealized Loss Position Less Than 12 Months [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7	[3]	56	[3]
Gross Unrealized Losses	0	[3]	(1)	[3]
Unrealized Loss Position Less Than 12 Months [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	18		76
Gross Unrealized Losses	0		(1)
Unrealized Loss Position Less Than 12 Months [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	6		0
Gross Unrealized Losses	0		0
Unrealized Loss Position Less Than 12 Months [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	239		259
Gross Unrealized Losses	(5)		(24)
Unrealized Loss Position Greater Than 12 Months [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	0	[1]	0	[1]
Gross Unrealized Losses	0	[1]	0	[1]
Unrealized Loss Position Greater Than 12 Months [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	2	[2]	3	[2]
Gross Unrealized Losses	0	[2]	0	[2]
Unrealized Loss Position Greater Than 12 Months [Member] | Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	3	[3]	4	[3]
Gross Unrealized Losses	0	[3]	(1)	[3]
Unrealized Loss Position Greater Than 12 Months [Member] | Total Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	5		7
Gross Unrealized Losses	0		(1)
Unrealized Loss Position Greater Than 12 Months [Member] | Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	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]
Fair Value	5		7
Gross Unrealized Losses	$ 0		$ (1)

[1] Equity Securities—Represent investments primarily in common stock within a broad range of industries and sectors. The unrealized losses are distributed over two hundred companies with limited impairment durations.

[2] Debt Securities (Government)—Unrealized losses on investments in United States Treasury obligations and Federal Agency mortgage-backed securities were caused by interest rate changes. Since these investments are guaranteed by the United States government or an agency of the United States government, it is not expected that these securities will settle for less than their amortized cost basis. Power does not intend to sell nor will it be more-likely-than-not required to sell these securities.

[3] Debt Securities (Corporate)—Represent investment grade corporate bonds which are not expected to settle for less than their amortized cost. Power does not intend to sell nor will it be more-likely-than-not required to sell these securities.