Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) - Interest Rate Contract - Lender Concentration Risk - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	$ 1,712	$ 1,113
Pass
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	1,700	1,052
Risk Level, Special Mention
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	5	59
Risk Level, Substandard
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	$ 7	$ 2