Summary of Short-Term Borrowings and Weighted-Average Rates (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Short-term borrowings, amount	$ 346	$ 279
Short-term borrowings, average	345	291
Short-term borrowings, maximum month-end balance	451	422
Short-term borrowngs, rate	0.04%	0.18%
Short-term borrowings, average rate	0.11%	0.17%
Other Short Term Borrowings [Member]
Short-term Debt [Line Items]
Short-term borrowings, amount	3,239	1,574
Short-term borrowings, average	2,777	1,635
Short-term borrowings, maximum month-end balance	$ 4,894	$ 1,975
Short-term borrowngs, rate	0.09%	0.14%
Short-term borrowings, average rate	0.12%	0.21%