Short-Term Borrowings (Summary of Short-Term Borrowings and Weighted-Average Rates) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Short Term Debt
Federal funds purchased	$ 132	$ 151
Other short-term borrowings	3,535	1,507
Federal Funds Purchased
Short Term Debt
Federal funds purchased	132	151
Short-term borrowings, average	506	920
Short-term borrowings, maximum month-end balance	$ 739	$ 200
Short-term borrowngs, rate	0.61%	0.30%
Short-term borrowings, average rate	0.39%	0.13%
Other Short Term Borrowings
Short Term Debt
Other short-term borrowings	$ 3,535	$ 1,507
Short-term borrowings, average	2,845	1,721
Short-term borrowings, maximum month-end balance	$ 6,374	$ 4,904
Short-term borrowngs, rate	0.54%	0.11%
Short-term borrowings, average rate	0.36%	0.12%