Summary of Short-Term Borrowings and Weighted-Average Rates (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Short-term borrowings, amount	$ 284	$ 901
Short-term borrowings, average	503	560
Short-term borrowings, maximum month-end balance	925	901
Short-term borrowngs, rate	0.03%	0.10%
Short-term borrowings, average rate	0.12%	0.14%
Other Short Term Borrowings [Member]
Short-term Debt [Line Items]
Short-term borrowings, amount	1,380	6,280
Short-term borrowings, average	3,024	4,246
Short-term borrowings, maximum month-end balance	$ 8,001	$ 6,330
Short-term borrowngs, rate	0.09%	0.15%
Short-term borrowings, average rate	0.18%	0.18%