Derivative Instruments and Hedging Activities - Commodity Derivative Activity in Accumulated Other Comprehensive Loss (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 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			20	(119)
Net change in derivative fair value, Before tax				171
Ineffectiveness reclassified into earnings, Before tax
Unrealized gain (loss) on derivatives at end of period, Before tax	10	9	10	9
Unrealized gain (loss) on derivatives at beginning of period, After tax			(6)	114
Realized amounts reclassified into earnings, After tax			14	(92)
Net change in derivative fair value, After tax	7	111	(1)	112
Ineffectiveness reclassified into earnings, After tax
Unrealized gain on derivatives at end of period, After tax	$ 7	$ 134	$ 7	$ 134