Regulatory Assets, Liabilities, And Balancing Accounts (Long-Term Regulatory Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	$ 5,008	$ 4,733
Cost Of Removal Obligations [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	3,555	3,460
Recoveries In Excess Of AROs [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	669	611
Public Purpose Programs [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	607	499
Other Long-Term Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	$ 177	$ 163