Discontinued Operations - Income Recognized from Cross-Border Energy Lease Investments (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
Schedule of Discontinued Operations [Line Items]
Income tax expense, income from operations of discontinued operations	$ 1	$ 5	$ 3	$ 17
Cross-Border Energy Lease Investments [Member]
Schedule of Discontinued Operations [Line Items]
Income tax expense, income from operations of discontinued operations	0	1	(44)	3
Income tax expense, Net gains associated with the accelerated disposition of retail electric and natural gas contracts	$ 4	$ 30	$ (1)	$ 30