Derivative Instruments and Hedging Activities - Commodity Derivative Activity in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Unrealized gain (loss) on derivatives at beginning of period, Before tax	$ (10)	$ 145
Realized amounts reclassified into earnings, Before tax	9	(41)
Net change in derivative fair value, Before tax	(11)	(29)
Ineffectiveness reclassified into earnings, Before tax		1
Unrealized gain on derivatives at end of period, Before tax	(12)	76
Unrealized gain (loss) on derivatives at beginning of period, After tax	(6)	114
Realized amounts reclassified into earnings, After tax	6	(34)
Net change in derivative fair value, After tax	(8)	1
Ineffectiveness reclassified into earnings, After tax
Unrealized gain on derivatives at end of period, After tax	$ (8)	$ 81