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, 2011	Dec. 31, 2010
Notional amount of the risk participation agreements	$ 808	$ 851
Pass
Notional amount of the risk participation agreements	772	744
Risk Level, Special Mention
Notional amount of the risk participation agreements	14	37
Risk Level, Substandard
Notional amount of the risk participation agreements	18	69
Risk Level, Doubtful
Notional amount of the risk participation agreements	$ 4	$ 1