Investment Securities - Amount of Gross Gains and Losses Realized through Sales of Available-for-Sale Investment Securities (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt Securities, Available-for-sale, Realized Gain (Loss), Excluding Other-than-temporary Impairment [Abstract]
Realized gains	$ 36	$ 10	$ 41	$ 15
Realized losses	(19)		(19)
Net realized gains (losses)	17	10	22	15
Income tax (benefit) on net realized gains (losses)	$ 4	$ 3	$ 6	$ 4