Regulatory Assets, Liabilities, And Balancing Accounts (Long-Term Regulatory Assets) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 6,534	$ 6,506
Pension Benefits [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	2,978	2,899
Deferred Income Taxes [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	1,520	1,444
Utility Retained Generation [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	582	613
Environmental Compliance Costs [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	557	520
Price Risk Management [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	259	339
Electromechanical Meters [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	221	247
Unamortized Loss, Net Of Gain, On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	152	163
Other Long-Term Regulatory Assets [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 265	$ 281