Short-term Borrowings - Summary of Short-Term Borrowings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Short-term borrowings	$ 2,198	$ 3,518
Trading Liabilities
Short-term Debt [Line Items]
Average balance	457	503	$ 683
Short-term borrowings	353	506	335
Maximum month-end outstanding	$ 983	$ 754	$ 891
Average rate for the year	1.24%	2.48%	2.83%
Average rate at year-end	0.77%	2.07%	3.21%
Federal Funds Purchased
Short-term Debt [Line Items]
Average balance	$ 862	$ 738	$ 405
Short-term borrowings	845	548	257
Maximum month-end outstanding	$ 1,487	$ 1,282	$ 503
Average rate for the year	0.34%	2.08%	1.89%
Average rate at year-end	0.10%	1.55%	2.50%
Securities Sold Under Agreements to Repurchase
Short-term Debt [Line Items]
Average balance	$ 1,109	$ 701	$ 714
Short-term borrowings	1,187	717	763
Maximum month-end outstanding	$ 1,661	$ 772	$ 891
Average rate for the year	0.46%	1.89%	1.40%
Average rate at year-end	0.26%	1.72%	1.66%
Other Short-term Borrowings
Short-term Debt [Line Items]
Average balance	$ 626	$ 538	$ 1,047
Short-term borrowings	166	2,253	115
Maximum month-end outstanding	$ 4,061	$ 2,276	$ 2,229
Average rate for the year	0.92%	2.34%	1.82%
Average rate at year-end	0.09%	2.14%	2.48%