Regulatory Assets, Liabilities, And Balancing Accounts (Long-Term Regulatory Assets) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 6,827	$ 6,809
Pension Benefits [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	3,356	3,275
Deferred Income Taxes [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	1,772	1,627
Utility Retained Generation [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	515	552
Environmental Compliance Costs [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	609	604
Price Risk Management [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	144	210
Electromechanical Meters [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	150	194
Unamortized Loss, Net Of Gain, On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	140	141
Other Long-Term Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 141	$ 206