Derivative Instruments and Hedging Activities - Commodity Derivative Activity in Accumulated Other Comprehensive Loss (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 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			18	(202)
Net change in derivative fair value, Before tax			(7)	97
Ineffectiveness reclassified into earnings, Before tax			1	(1)
Unrealized gain (loss) on derivatives at end of period, Before tax	2	39	2	39
Unrealized gain (loss) on derivatives at beginning of period, After tax			(6)	113
Realized amounts reclassified into earnings, After tax			13	(151)
Net change in derivative fair value, After tax	(5)	(41)	(6)	71
Ineffectiveness reclassified into earnings, After tax	1		1
Unrealized gain on derivatives at end of period, After tax	$ 2	$ 33	$ 2	$ 33