Stockholders' Equity (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Net income (loss)
Net income (loss) attributable to common stockholders	$ 1,087		$ 182		$ (1,355)		$ 1,571
Less distributions on participating securities	2		0		3		1
Less undistributed income allocated to participating securities	6		1		0		9
Basic	1,079		181		(1,358)		1,561
Diluted	$ 1,079		$ 181		$ (1,358)		$ 1,561
Shares
Average number of common shares outstanding—basic	506		503		505		502
Dilutive effect of stock options	2		2		0		2
Average number of common shares outstanding—diluted	508		505		505		504
Excluded (1)	3	[1]	3	[1]	11	[1]	4	[1]
Net income (loss) per common share
Basic	$ 2.13		$ 0.36		$ (2.69)		$ 3.11
Diluted	$ 2.12		$ 0.36		$ (2.69)		$ 3.10
Dividends per common share	$ 0.27		$ 0.18		$ 0.72		$ 0.36

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