New And Significant Accounting Policies (Narrative) (Detail) (USD $) In Millions, unless otherwise specified			3 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Lease Payments And Investment Contributions [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Payments made under tax equity agreements			$ 363
Carrying amount of investment in tax equity agreements	$ 138	$ 166
Number of tax equity agreements	4