Short-Term Borrowings (Summary of Short-Term Borrowings and Weighted-Average Rates) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Short Term Debt
Federal funds purchased	$ 174	$ 132
Other short-term borrowings	4,012	3,535
Federal Funds Purchased
Short Term Debt
Federal funds purchased	174	132
Short-term borrowings, average	557	506
Short-term borrowings, maximum month-end balance	$ 1,495	$ 739
Short-term borrowngs, rate	1.37%	0.61%
Short-term borrowings, average rate	1.01%	0.39%
Other Short Term Borrowings
Short Term Debt
Other short-term borrowings	$ 4,012	$ 3,535
Short-term borrowings, average	3,158	2,845
Short-term borrowings, maximum month-end balance	$ 6,307	$ 6,374
Short-term borrowngs, rate	1.28%	0.54%
Short-term borrowings, average rate	0.96%	0.36%