Regulatory Assets, Liabilities, And Balancing Accounts (Long-Term Regulatory Assets) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 5,217	$ 4,913
Pension Benefits [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	1,397	1,444
Deferred Income Taxes [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	2,269	1,835
Utility Retained Generation [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	468	503
Environmental Compliance Costs [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	714	628
Price Risk Management [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	83	106
Electromechanical Meters [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	87	135
Unamortized Loss, Net Of Gain, On Reacquired Debt [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	118	135
Other Long Term Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Total long-term regulatory assets	$ 81	$ 127