Risk Management and Trading Activities - Pre-Tax Gains (Losses) from Trading Activities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax unrealized and realized gains (losses) from trading activities	$ 14	$ 85	$ 70	$ 94
Commodity [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax unrealized and realized gains (losses) from trading activities	6	75	53	83
Foreign exchange [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax unrealized and realized gains (losses) from trading activities	(1)	1		3
Other [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Pre-tax unrealized and realized gains (losses) from trading activities	$ 9	$ 9	$ 17	$ 8