- Revenues and Expenses with Affiliates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Securities borrowed or purchased under resale agreements	$ 190	$ 765	$ 652
Interest Expense, Deposits	363	826	441
Long-term debt	349	497	365
Securities loaned or sold under repurchase agreements	155	830	734
Trading account activities	38	70	(24)
Fees from affiliates	1,489	1,439	1,213
Other	247	248	230
Other Noninterest Expense	386	422	409
MUSA and Various other Non-Bank Subsidiaries | Affiliated Entity
Related Party Transaction [Line Items]
Securities borrowed or purchased under resale agreements	19	66	66
Interest Income, Other	1	2	2
Interest Expense, Deposits	3	13	3
Commercial paper and other short-term borrowings	5	3	2
Long-term debt	118	237	171
Securities loaned or sold under repurchase agreements	4	25	18
Trading account activities	(56)	34	0
Fees from affiliates	1,489	1,439	1,213
Other	32	(6)	1
Other Noninterest Expense	$ 97	$ 96	$ 115