Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) (Interest Rate Contract, Lender Concentration Risk, USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Notional amount of the risk participation agreements	$ 1,006	$ 808
Pass
Notional amount of the risk participation agreements	993	772
Risk Level, Special Mention
Notional amount of the risk participation agreements		14
Risk Level, Substandard
Notional amount of the risk participation agreements	13	18
Risk Level, Doubtful
Notional amount of the risk participation agreements		$ 4