Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Parenthetical) (Detail)	Sep. 30, 2012	Dec. 31, 2011
Federal funds purchased and securities sold under repurchase agreements [Member]
Schedule Of Commercial Paper And Short Term Borrowings [Line Items]
commercial paper and other short-term borrowings, Weighted average interest rates	0.25%	0.06%
Commercial paper [Member]
Schedule Of Commercial Paper And Short Term Borrowings [Line Items]
commercial paper and other short-term borrowings, Weighted average interest rates	0.24%	0.22%
Term federal funds purchased [Member]
Schedule Of Commercial Paper And Short Term Borrowings [Line Items]
commercial paper and other short-term borrowings, Weighted average interest rates	0.18%	0.15%
Fixed and floating rate Federal Home Loan Bank advances with maturities ranging from January 2013 to February 2016 [Member]
Schedule Of Commercial Paper And Short Term Borrowings [Line Items]
commercial paper and other short-term borrowings, Weighted average interest rates		0.48%
Other borrowed funds [Member]
Schedule Of Commercial Paper And Short Term Borrowings [Line Items]
commercial paper and other short-term borrowings, Weighted average interest rates		0.73%