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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Tax Expense [Member]
Discontinued Operations [Line Items]
Income tax expense, income from operations of discontinued operations	$ 0	$ 6	$ 1	$ 11
Income tax expense, Net gains associated with the accelerated disposition of retail electric and natural gas contracts	$ 1	$ 0	$ 0	$ 0