Computation of Earnings Per Share (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
Basic earnings (loss) per share:
Net income (loss)	$ 469	$ 339	$ 52	$ (109)
Less: Income allocable to participating securities	(1)	(1)
Earnings (loss) available to common stockholders	468	338	52	(109)
Basic weighted average shares outstanding	341	331	337	331
Earnings (loss) per share, basic	$ 1.37	$ 1.02	$ 0.15	$ (0.33)
Diluted earnings (loss) per share:
Earnings (loss) available to common stockholders	468	338	52	(109)
Effect of convertible notes on diluted earnings per share, amount	11	11
Earnings (loss) available to common stockholders including the effect of dilutive securities	$ 479	$ 349	$ 52	$ (109)
Diluted shares outstanding:
Basic weighted-average shares outstanding	341	331	337	331
Effect of convertible notes on diluted earnings per share, shares	53	61
Effect of employee stock options		1
Diluted weighted-average shares outstanding	394	393	337	331
Earnings (loss) per share, diluted	$ 1.21	$ 0.89	$ 0.15	$ (0.33)
Restricted Stock And Stock Options
Potentially dilutive shares excluded from diluted per share amounts:
Potentially dilutive shares excluded from diluted per share amounts	3	5	3	7
Convertible Notes
Potentially dilutive shares excluded from diluted per share amounts:
Potentially dilutive shares excluded from diluted per share amounts			57	61