Computation of Earnings (Loss) Per Share (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic earnings (loss) per share:
Earnings (loss) available to common stockholders									$ 1,132	$ 571	$ (723)
Basic weighted-average shares outstanding									371	348	331
Earnings (loss) per share, basic	$ 0.08	$ 2.49	$ 2.11	$ (1.66)	$ 0.39	$ 1.06	$ 1.37	$ (1.26)	$ 3.05	$ 1.64	$ (2.18)
Diluted earnings (loss) per share:
Earnings (loss) available to common stockholders									1,132	571	(723)
Effect of dilutive securities									11	26
Earnings (loss) available to common stockholders including the effect of dilutive securities									$ 1,143	$ 597	$ (723)
Diluted shares outstanding:
Basic weighted-average shares outstanding									371	348	331
Effect of convertible notes									18	42
Effect of restricted stock and employee stock options									1	1
Diluted weighted-average shares outstanding									390	391	331
Earnings (loss) per share, diluted	$ 0.07	$ 2.37	$ 2.01	$ (1.66)	$ 0.37	$ 0.98	$ 1.21	$ (1.26)	$ 2.93	$ 1.53	$ (2.18)
Potentially dilutive shares excluded from diluted per share amounts:
Potentially dilutive shares excluded from diluted per share amounts									4	7	66
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	5
Convertible Notes
Potentially dilutive shares excluded from diluted per share amounts:
Potentially dilutive shares excluded from diluted per share amounts									3	4	61