Summary of Short-Term Borrowings and Weighted-Average Rates (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Short Term Debt [Line Items]
Federal funds purchased	$ 151	$ 144
Other short-term borrowings	1,507	1,556
Federal Funds Purchased
Short Term Debt [Line Items]
Federal funds purchased	151	144
Short-term borrowings, average	920	458
Short-term borrowings, maximum month-end balance	$ 200	$ 286
Short-term borrowngs, rate	0.30%	0.08%
Short-term borrowings, average rate	0.13%	0.09%
Other Short Term Borrowings
Short Term Debt [Line Items]
Other short-term borrowings	$ 1,507	$ 1,556
Short-term borrowings, average	1,721	1,873
Short-term borrowings, maximum month-end balance	$ 4,904	$ 3,756
Short-term borrowngs, rate	0.11%	0.08%
Short-term borrowings, average rate	0.12%	0.10%