Derivative Instruments and Hedging Activities - Commodity Derivative Activity in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) on derivatives at beginning of period, Before tax	$ 145	$ (54)	$ (267)
Realized amounts reclassified into earnings, Before tax	(268)	13	(165)
Net change in derivative fair value, Before tax	113	188	376
Ineffectiveness reclassified into earnings, Before tax		(2)	2
Unrealized gain on derivatives at end of period, Before tax	(10)	145	(54)
Unrealized gain (loss) on derivatives at beginning of period, After tax	(6)	114	(19)
Realized amounts reclassified into earnings, After tax	(199)	19	(106)
Net change in derivative fair value, After tax	79	115	256
Ineffectiveness reclassified into earnings, After tax		(1)	1
Unrealized gain on derivatives at end of period, After tax	$ (6)	$ 114	$ (19)