Stockholders' Equity (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Net income (loss)
Net income (loss) attributable to common stockholders	$ 227		$ 929		$ (2,442)		$ 1,389
Less distributions on participating securities	1		1		1		1
Less undistributed income allocated to participating securities	0		5		0		8
Basic	226		923		(2,443)		1,380
Diluted	$ 226		$ 923		$ (2,443)		$ 1,380
Shares
Average number of common shares outstanding—basic	505		502		505		501
Dilutive effect of stock options	2		2		0		3
Average number of common shares outstanding—diluted	507		504		505		504
Excluded (1)	4	[1]	5	[1]	11	[1]	4	[1]
Net income (loss) per common share
Basic	$ 0.45		$ 1.84		$ (4.84)		$ 2.75
Diluted	$ 0.45		$ 1.83		$ (4.84)		$ 2.74
Dividends per common share	$ 0.27		$ 0.09		$ 0.45		$ 0.18

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