Schedule of Details Supporting Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income available for common stockholders	$ 101	$ 314	$ 431	$ 430	$ 145	$ 290	$ 344	$ 368	$ 1,276	$ 1,147	$ 1,231
Weighted-average outstanding shares of common stock used to compute basic earnings per common share									149,455	154,187	157,503
Shares used to compute diluted earnings per common share									151,142	155,874	159,151
Basic earnings per common share (in dollars per share)	$ 0.68	$ 2.11	$ 2.88	$ 2.86	$ 0.96	$ 1.87	$ 2.22	$ 2.37	$ 8.54	$ 7.44	$ 7.81
Diluted earnings per common share (in dollars per share)	$ 0.67	$ 2.09	$ 2.85	$ 2.82	$ 0.94	$ 1.85	$ 2.19	$ 2.35	$ 8.44	$ 7.36	$ 7.73
Number of antidilutive stock options and restricted stock awards excluded from computation									415	320	704
Employee stock options
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of employee stock options and restricted stock									192	230	322
Restricted stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of employee stock options and restricted stock									1,495	1,457	1,326