Financial Risk Management and Trading Activities - Pre-Tax Unrealized and Realized Gains (Losses) from Trading Activities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments Gain Loss [Line Items]
Pre-tax unrealized and realized gains (losses) from trading activities	$ 68	$ 14	$ 182	$ 70
Commodity [Member]
Derivative Instruments Gain Loss [Line Items]
Pre-tax unrealized and realized gains (losses) from trading activities	52	6	147	53
Foreign Exchange [Member]
Derivative Instruments Gain Loss [Line Items]
Pre-tax unrealized and realized gains (losses) from trading activities	2	(1)	1
Equity and Other [Member]
Derivative Instruments Gain Loss [Line Items]
Pre-tax unrealized and realized gains (losses) from trading activities	$ 14	$ 9	$ 34	$ 17