Discontinued Operations (Operating Results For Retail Natural Gas Supply Business) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Discontinued Operations And Disposal Groups [Abstract]
Income from operations of discontinued operations, net of income taxes	$ 2	$ 5
Unrealized losses associated with retail natural gas supply contracts, net of income taxes	(2)
Income From Discontinued Operations, Net of Income Taxes		$ 5

After tax income (loss) from operations of a business component (exclusive of any gain (loss) on disposal, or provision therefore) during the reporting period, until its disposal.

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Amount of income (loss) from a disposal group, net of income tax before extraordinary items allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal.

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