Stockholders' Equity (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012
Net income (loss)
Net income (loss) attributable to common stockholders	$ 182		$ 121		$ 1,571		$ 2,188
Less distributions on participating securities	0		0		1		1
Less undistributed income allocated to participating securities	1		1		9		13
Basic	181		120		1,561		2,174
Diluted	$ 181		$ 120		$ 1,561		$ 2,174
Shares
Average number of common shares outstanding—basic	503		500		502		499
Dilutive effect of stock options	2		2		2		2
Average number of common shares outstanding—diluted	505		502		504		501
Excluded (1)	3	[1]	6	[1]	4	[1]	6	[1]
Net income (loss) per common share
Basic	$ 0.36		$ 0.24		$ 3.11		$ 4.35
Diluted	$ 0.36		$ 0.24		$ 3.10		$ 4.34
Dividends per common share	$ 0.18		$ 0.09		$ 0.36		$ 0.27

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