Short-Term Borrowings - Components of Short-Term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Balance at year end, Securities Sold Under Repurchase Agreements	$ 0	$ 0
Federal Funds Purchased [Member]
Short-term Debt [Line Items]
Balance at year end, Federal Funds Purchase	200	0	$ 3
Average during the year	61	537	128
Maximum month-end balance	$ 1,000	$ 3,197	$ 2,331
Weighted-average rate during the year	2.12%	1.68%	0.72%
Weighted-average rate at December 31	1.56%	0.00%	0.00%
Securities Sold under Agreements to Repurchase [Member]
Short-term Debt [Line Items]
Average during the year	$ 203	$ 391	$ 389
Maximum month-end balance	$ 283	$ 614	$ 472
Weighted-average rate during the year	0.22%	0.09%	0.08%
Weighted-average rate at December 31	0.09%	0.09%	0.08%
Balance at year end, Securities Sold Under Repurchase Agreements	$ 187	$ 319	$ 374
Other Short Term Borrowings [Member]
Short-term Debt [Line Items]
Average during the year	730	915	1,140
Maximum month-end balance	$ 847	$ 1,133	$ 1,242
Weighted-average rate during the year	2.31%	2.34%	1.34%
Weighted-average rate at December 31	1.99%	2.92%	2.01%
Balance at year end, Other Short-Term Borrowings	$ 705	$ 544	$ 634