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	Dec. 31, 2012		Dec. 31, 2011
Unrealized Loss Position Less Than 12 Months [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 204		$ 259
Gross Unrealized Losses	(6)		(24)
Unrealized Loss Position Less Than 12 Months [Member] | Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	139	[1]	183	[1]
Gross Unrealized Losses	(6)	[1]	(23)	[1]
Unrealized Loss Position Less Than 12 Months [Member] | Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	34	[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	31	[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	65		76
Gross Unrealized Losses			(1)
Unrealized Loss Position Greater Than 12 Months [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	7		7
Gross Unrealized Losses			(1)
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	1	[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	6	[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	7		7
Gross Unrealized Losses			$ (1)

[1] Equity Securities—Investments in marketable equity securities within the NDT Fund are primarily in common stocks within a broad range of industries and sectors. The unrealized losses are distributed over hundreds of companies with limited impairment durations. Power does not consider these securities to be other-than-temporarily impaired as of December 31, 2012.

[2] Debt Securities (Government)—Unrealized losses on Power’s NDT 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, since Power does not intend to sell nor will it be more-likely-than-not required to sell. Power does not consider these securities to be other-than-temporarily impaired as of December 31, 2012.

[3] Debt Securities (Corporate)—Power’s investments in corporate bonds are primarily in investment grade securities. It is not expected that these securities would settle for less than their amortized cost. Since Power does not intend to sell these securities nor will it be more-likely-than-not required to sell, Power does not consider these debt securities to be other-than-temporarily impaired as of December 31, 2012.