Investment Securities - Amount of Gross Gains and Losses Realized through Sales of Available-for-Sale Investment Securities (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt Securities, Available-for-sale, Realized Gain (Loss), Excluding Other-than-temporary Impairment [Abstract]
Realized gains	$ 5	$ 5
Realized losses	0	0
Net realized gains (losses)	5	5
Income tax (benefit) on net realized gains (losses)	$ 2	$ 1