Risk Management and Trading Activities - Net Pre-Tax Gains (Losses) on Derivative Contracts Used for Corporate Risk Management not Designated as Hedges (Detail) (Corporate Risk Management [Member], Not designated as hedges [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net pre-tax gains (losses) on derivative contracts not designated as hedges
Net pre-tax gains (losses) on derivative contracts	$ 44	$ (14)	$ (14)
Commodity [Member]
Net pre-tax gains (losses) on derivative contracts not designated as hedges
Net pre-tax gains (losses) on derivative contracts	1	1	(7)
Foreign exchange [Member]
Net pre-tax gains (losses) on derivative contracts not designated as hedges
Net pre-tax gains (losses) on derivative contracts	$ 43	$ (15)	$ (7)