Regulatory Assets, Liabilities, And Balancing Accounts (Long-Term Regulatory Liabilities) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	$ 5,187	$ 5,088
Cost Of Removal Obligations [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	3,709	3,625
Recoveries In Excess Of AROs [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	673	620
Public Purpose Programs [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	539	590
Other Long-Term Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Total long-term regulatory liabilities	$ 266	$ 253