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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments [Abstract]
Realized gains	$ 0	$ 3	$ 1	$ 8
Realized losses	0		(1)
Net realized gains (losses)	0	3	0	8
Income tax (benefit) on net realized gains (losses)	$ 0	$ 1	$ 0	$ 3