Short-Term Borrowings (Summary of Short-Term Borrowings and Weighted-Average Rates) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Federal funds purchased	$ 300	$ 260
Other short-term borrowings	1,192	1,011
Federal Funds Purchased
Short-term Debt [Line Items]
Short-term borrowings, average	385	1,267
Short-term borrowings, maximum month-end balance	$ 1,625	$ 2,693
Short-term borrowngs, rate	0.14%	1.49%
Short-term borrowings, average rate	0.58%	2.26%
Other Short Term Borrowings
Short-term Debt [Line Items]
Short-term borrowings, average	$ 1,709	$ 1,046
Short-term borrowings, maximum month-end balance	$ 4,542	$ 4,046
Short-term borrowngs, rate	0.19%	1.24%
Short-term borrowings, average rate	0.81%	2.67%