Discontinued Operations - Operating Results for Retail Natural Gas Supply Business (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Discontinued Operations And Disposal Groups [Abstract]
Operating revenue	$ 34	$ 109	$ 84	$ 265
Income from operations of discontinued operations, net of income taxes	1	9	3	17
Gain associated with retail natural gas supply contracts, net of income taxes	3		1
Income From Discontinued Operations, Net of Income Taxes	$ 4	$ 9	$ 4	$ 17

Amount of gain (loss), after tax expense or benefit and not previously recognized, resulting from the sale of a business component.

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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 sales or other form of revenues attributable to the disposal group, including a component of the entity (discontinued operation), during the reporting period.

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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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