Stockholders' Equity (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Net income (loss)
Net income (loss) attributable to common stockholders	$ 460		$ 2,156
Less undistributed income allocated to participating securities	3		12
Basic	457		2,144
Diluted	$ 457		$ 2,144
Shares
Average number of common shares outstanding - basic	501		499
Dilutive effect of stock options	2		2
Average number of common shares outstanding - diluted	503		501
Excluded	4	[1]	4	[1]
Net income (loss) per common share
Basic	$ 0.91		$ 4.30
Diluted	$ 0.91		$ 4.28
Dividends per common share	$ 0.09		$ 0.09

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