Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Notional amount of the risk participations agreements	$ 1,113	$ 1,203
Pass
Derivatives, Fair Value [Line Items]
Notional amount of the risk participations agreements	1,052	1,153
Risk Level, Special Mention
Derivatives, Fair Value [Line Items]
Notional amount of the risk participations agreements	59	38
Risk Level, Substandard
Derivatives, Fair Value [Line Items]
Notional amount of the risk participations agreements	$ 2	$ 12