Computation of Earnings Per Share (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
Basic earnings per share:
Net income	$ 789	$ 469	$ 180	$ 52
Less: Income allocable to participating securities	(1)	(1)	(1)
Earnings available to common stockholders	788	468	179	52
Basic weighted-average shares outstanding	373	341	371	337
Earnings per share, basic	$ 2.11	$ 1.37	$ 0.48	$ 0.15
Diluted earnings per share:
Earnings available to common stockholders	788	468	179	52
Effect of convertible notes	6	11	6
Earnings available to common stockholders including the effect of dilutive securities	$ 794	$ 479	$ 185	$ 52
Diluted shares outstanding:
Basic weighted-average shares outstanding	373	341	371	337
Effect of convertible notes	22	53	21
Effect of employee stock options	1
Diluted weighted-average shares outstanding	396	394	392	337
Earnings per share, diluted	$ 2.01	$ 1.21	$ 0.47	$ 0.15
Restricted Stock And Stock Options
Potentially dilutive shares excluded from diluted per share amounts:
Potentially dilutive shares excluded from diluted per share amounts	1	3	2	3
Convertible Notes
Potentially dilutive shares excluded from diluted per share amounts:
Potentially dilutive shares excluded from diluted per share amounts			4	57