Risk Management and Trading Activities - Energy Marketing Risk Management Activities - Additional Information (Detail) (Energy Marketing Risk Management [Member], USD $) In Millions, unless otherwise specified			3 Months Ended		9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Not designated as hedges [Member]	Sep. 30, 2012 Not designated as hedges [Member]	Sep. 30, 2013 Not designated as hedges [Member]	Sep. 30, 2012 Not designated as hedges [Member]
Derivative [Line Items]
Net realized and unrealized pre-tax gains and losses on derivative contracts not designated as hedges			$ (125)	$ 66	$ (49)	$ 145
After-tax deferred losses in Accumulated other comprehensive income (loss)	$ 9	$ 22