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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Interest Income, Securities, Operating, by Taxable Status [Abstract]
Taxable	$ 471	$ 449	$ 936	$ 885
Non-taxable	52	56	104	115
Total interest income from investment securities	$ 523	$ 505	$ 1,040	$ 1,000