Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) (Interest Rate Contract, Lender Concentration Risk, USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Notional amount of the risk participation agreements	$ 957	$ 808	$ 723
Pass
Notional amount of the risk participation agreements	914	772	645
Risk Level, Special Mention
Notional amount of the risk participation agreements		14	34
Risk Level, Substandard
Notional amount of the risk participation agreements	43	18	43
Risk Level, Doubtful
Notional amount of the risk participation agreements		4
Unlikely to be Collected Financing Receivable
Notional amount of the risk participation agreements			$ 1