Commitments And Contingencies (Third-Party Power Purchases) (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Long-term Purchase Commitment [Line Items]
2012	$ 608,000,000
2013	3,075,000,000
2014	3,405,000,000
2015	3,418,000,000
2016	3,287,000,000
Thereafter	39,341,000,000
Total	53,134,000,000
Cost incurred by the Utility under power purchase agreements	1,700,000,000	1,800,000,000
Total minimum lease payments terminated	136,000,000
Capital leases, future minimum lease payments	$ 125,000,000
Minimum [Member]
Long-term Purchase Commitment [Line Items]
Long-term agreements range, years	10
Maximum [Member]
Long-term Purchase Commitment [Line Items]
Long-term agreements range, years	25