Short-Term Borrowings (Summary of Short-Term Borrowings and Weighted-Average Rates) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Federal funds purchased	$ 260	$ 1,925
Other short-term borrowings	1,011	573
Federal Funds Purchased
Short-term Debt [Line Items]
Federal funds purchased	260	1,925
Short-term borrowings, average	1,267	1,509
Short-term borrowings, maximum month-end balance	$ 2,693	$ 2,684
Short-term borrowngs, rate	1.49%	2.40%
Short-term borrowings, average rate	2.26%	1.97%
Other Short Term Borrowings
Short-term Debt [Line Items]
Other short-term borrowings	$ 1,011	$ 573
Short-term borrowings, average	1,046	1,611
Short-term borrowings, maximum month-end balance	$ 4,046	$ 6,313
Short-term borrowngs, rate	1.24%	1.95%
Short-term borrowings, average rate	2.67%	1.82%