Discontinued Operations - Operating Results for Retail Natural Gas Supply Business (Parenthetical) (Detail) (Income Tax Expense [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Tax Expense [Member]
Discontinued Operations [Line Items]
Income tax expense, income from operations of discontinued operations	$ 1	$ 5	$ 3	$ 17

Income tax expense or benefit attributable to Income or Loss from discontinued operations during the phase-out period.

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