Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule Of Commercial Paper And Short Term Borrowings [Line Items]
Federal funds purchased and securities sold under repurchase agreements with weighted average interest rates of 0.25% and 0.06% at September 30, 2012 and December 31, 2011, respectively	$ 119	$ 597
Commercial paper, with weighted average interest rates of 0.24% and 0.22% at September 30, 2012 and December 31, 2011, respectively	1,702	2,498
Other borrowed funds:
Term federal funds purchased, with a weighted average interest rate of 0.18% and 0.15% at September 30, 2012 and December 31, 2011, respectively	270	50
Federal Home Loan Bank advances, with a weighted average interest rate of 0.48% at December 31, 2011		500
All other borrowed funds, with a weighted average interest rate of 0.73% at December 31, 2011		38
Total commercial paper and other short-term borrowings	$ 2,091	$ 3,683