Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) (Interest Rate Contract, Lender Concentration Risk, USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	$ 1,237	$ 1,203
Pass
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	1,147	1,153
Risk Level, Special Mention
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	50	38
Risk Level, Substandard
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	$ 40	$ 12