Regulatory Assets, Liabilities, And Balancing Accounts (Long-Term Regulatory Assets) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 6,527	$ 6,506
Pension Benefits [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	3,019	2,899
Deferred Income Taxes [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	1,584	1,444
Utility Retained Generation [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	567	613
Environmental Compliance Costs [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	576	520
Price Risk Management [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	223	339
Electromechanical Meters [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	207	247
Unamortized Loss, Net Of Gain, On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	147	163
Other Long-Term Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 204	$ 281