Summary of Short-Term Borrowings and Weighted-Average Rates (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Short-term borrowings, amount	$ 901	$ 346
Short-term borrowings, average	560	345
Short-term borrowings, maximum month-end balance	901	451
Short-term borrowngs, rate	0.10%	0.04%
Short-term borrowings, average rate	0.14%	0.11%
Other Short Term Borrowings [Member]
Short-term Debt [Line Items]
Short-term borrowings, amount	6,280	3,239
Short-term borrowings, average	4,246	2,777
Short-term borrowings, maximum month-end balance	$ 6,330	$ 4,894
Short-term borrowngs, rate	0.15%	0.09%
Short-term borrowings, average rate	0.18%	0.12%