Investment Securities - Summary of Information about Amount of Gross Gains and Losses Realized through Sales of Available-for-sale Investment Securities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Investments Debt And Equity Securities [Abstract]
Realized gains	$ 5	$ 12
Realized losses
Net realized gains (losses)	5	12
Income tax (benefit) on net realized gains (losses)	$ 2	$ 5