Risk Ratings of the Notional Amount of Risk Participation Agreements (Detail) (Interest Rate Contract, USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Notional amount of the risk participation agreements	$ 1,176	$ 1,006
Lender Concentration Risk
Notional amount of the risk participation agreements	1,200	1,000
Lender Concentration Risk | Pass
Notional amount of the risk participation agreements	1,160	993
Lender Concentration Risk | Risk Level, Special Mention
Notional amount of the risk participation agreements	3
Lender Concentration Risk | Risk Level, Substandard
Notional amount of the risk participation agreements	$ 13	$ 13