Commercial Paper and Other Short-Term Borrowings - Summary of Commercial Paper and Other Short-Term Borrowings (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Short Term Borrowings [Abstract]
Federal funds purchased and securities sold under repurchase agreements with weighted average interest rates of 0.07% and 0.96% at June 30, 2013 and December 31, 2012, respectively	$ 137	$ 123
Commercial paper, with weighted average interest rates of 0.17% and 0.20% at June 30, 2013 and December 31, 2012, respectively	3,655	1,240
Other borrowed funds:
Total commercial paper and other short-term borrowings	$ 3,792	$ 1,363