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Investment Securities - 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 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Investments Debt And Equity Securities [Abstract]        
Realized gains $ 3 $ 9 $ 8 $ 21
Realized losses 0      
Net realized gains (losses) 3 9 8 21
Income tax (benefit) on net realized gains (losses) $ 1 $ 3 $ 3 $ 8