Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) (Interest Rate Contract, Lender Concentration Risk, USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Notional amount of the risk participation agreements	$ 971	$ 808	$ 722
Pass
Notional amount of the risk participation agreements	940	772	654
Risk Level, Special Mention
Notional amount of the risk participation agreements		14	9
Risk Level, Substandard
Notional amount of the risk participation agreements	31	18	54
Risk Level, Doubtful
Notional amount of the risk participation agreements		4	4
Unlikely to be Collected Financing Receivable
Notional amount of the risk participation agreements			$ 1