Investment Securities - Amount of Interest Income from Taxable and Non-Taxable Investment Securities (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Interest Income, Securities, Operating, by Taxable Status [Abstract]
Taxable	$ 449	$ 403	$ 885	$ 784
Non-taxable	56	58	115	118
Total interest income from investment securities	$ 505	$ 461	$ 1,000	$ 902