Available-For-Sale Securities (Narrative) (Detail) (USD $)	1 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Accounts Receivable		$ 18,000,000	$ 27,000,000
Accounts Payable		53,000,000	22,000,000
Rabbi Trust [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Realized Gains		6,000,000	0	31,000,000
Accounts Receivable		4,000,000
Accounts Payable		5,000,000
Rabbi Trust [Member] | Power [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After tax amount of net unrealized gains recognized in AOCI		6,000,000
Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Amount of other-than-temporary impairments		18,000,000
Decommissioning Liability, Noncurrent		348,000,000
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Realized Gains	59,000,000	153,000,000	144,000,000	119,000,000
Nuclear Decommissioning Trust (NDT) Fund [Member] | Power [Member] | Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
After tax amount of net unrealized gains recognized in AOCI		84,000,000
Minimum [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Decommissioning Costs Including Contingencies		2,200,000,000
Maximum [Member] | Nuclear Decommissioning Trust (NDT) Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Decommissioning Costs Including Contingencies		$ 2,400,000,000