Stockholders' Equity (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Net income (loss)
Net income (loss) attributable to common stockholders	$ 929		$ (89)		$ 1,389		$ 2,067
Less distributions on participating securities	1				1		1
Less undistributed income allocated to participating securities	5				8		12
Basic	923		(89)		1,380		2,054
Diluted	$ 923		$ (89)		$ 1,380		$ 2,054
Shares
Average number of common shares outstanding—basic	502		500		501		499
Dilutive effect of stock options	2				3		2
Average number of common shares outstanding—diluted	504		500		504		501
Excluded (1)	5	[1]	12	[1]	4	[1]	5	[1]
Net income (loss) per common share
Basic	$ 1.84		$ (0.18)		$ 2.75		$ 4.11
Diluted	$ 1.83		$ (0.18)		$ 2.74		$ 4.10
Dividends per common share	$ 0.09		$ 0.09		$ 0.18		$ 0.18

[1] Inclusion of certain shares would have had an anti-dilutive effect.