Investment Securities - Amount of Interest Income from Taxable and Non-Taxable Investment Securities (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Interest Income, Securities, Operating, by Taxable Status [Abstract]
Taxable	$ 448	$ 419	$ 1,333	$ 1,203
Non-taxable	54	57	169	175
Total interest income from investment securities	$ 502	$ 476	$ 1,502	$ 1,378