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, 2012	Dec. 31, 2011	Mar. 31, 2011
Notional amount of the risk participation agreements	$ 824	$ 808	$ 834
Pass
Notional amount of the risk participation agreements	781	772	756
Risk Level, Special Mention
Notional amount of the risk participation agreements	14	14	21
Risk Level, Substandard
Notional amount of the risk participation agreements	29	18	56
Risk Level, Doubtful
Notional amount of the risk participation agreements		$ 4	$ 1