Risk Management and Trading Activities - Energy Marketing Risk Management Activities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Energy Marketing Risk Management Activities [Member]	Dec. 31, 2012 Energy Marketing Risk Management Activities [Member]	Mar. 31, 2013 Energy Marketing Risk Management Activities [Member] Not designated as hedges [Member]	Mar. 31, 2012 Energy Marketing Risk Management Activities [Member] Not designated as hedges [Member]
Derivative [Line Items]
Net realized and unrealized pre-tax gains on derivative contracts not designated as hedges					$ 32	$ 55
The length of time over which the Corporation hedges exposure to variability in future cash flows (in number of years)	One year or less
Maximum duration for the outstanding hedging contracts (in number of years)	3 years
After-tax deferred losses in Accumulated other comprehensive income (loss)			18	22
Reclassification of accumulated other comprehensive income/(losses) into Income from discontinued operations over the next twelve months			(11)
Gain (loss) on ineffectiveness recognized immediately in Income from discontinued operations	$ 1	$ (1)