Regulatory Assets, Liabilities, And Balancing Accounts (Long-Term Regulatory Assets) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 6,412	$ 6,322
Pension Benefits [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	2,332	2,347
Deferred Income Taxes [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	2,521	2,390
Utility Retained Generation [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	446	456
Environmental Compliance Costs [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	674	717
Price Risk Management [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	170	127
Electromechanical Meters [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	53	70
Unamortized Loss, Net Of Gain, On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	108	113
Other Long Term Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 108	$ 102