Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) (Interest Rate Contract, Lender Concentration Risk, USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	$ 1,154	$ 1,113
Pass
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	1,093	1,052
Risk Level, Special Mention
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	59	59
Risk Level, Substandard
Derivatives, Fair Value [Line Items]
Notional amount of the risk participation agreements	$ 2	$ 2