Computation of Earnings Per Share (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
Basic earnings (loss) per share:
Net income (loss)	$ 379	$ 6	$ 431	$ (103)
Less: Income allocable to participating securities	(1)		(1)
Earnings (loss) available to common stockholders	378	6	430	(103)
Basic weighted average shares outstanding	357	331	343	331
Earnings (loss) per share, basic	$ 1.06	$ 0.02	$ 1.25	$ (0.31)
Diluted earnings (loss) per share:
Earnings (loss) available to common stockholders	378	6	430	(103)
Effect of convertible notes on diluted earnings per share, amount	9		20
Earnings (loss) available to common stockholders including the effect of dilutive securities	$ 387	$ 6	$ 450	$ (103)
Diluted shares outstanding:
Basic weighted-average shares outstanding	357	331	343	331
Effect of convertible notes on diluted earnings per share, shares	37		46
Effect of employee stock options	1	1	1
Diluted weighted-average shares outstanding	395	332	390	331
Earnings (loss) per share, diluted	$ 0.98	$ 0.02	$ 1.15	$ (0.31)
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	6
Convertible Notes
Potentially dilutive shares excluded from diluted per share amounts:
Potentially dilutive shares excluded from diluted per share amounts		61	4	61