- Revenues and Expenses with Affiliates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Securities borrowed or purchased under resale agreements	$ 652	$ 347	$ 195
Interest Expense, Deposits	441	241	194
Commercial paper and other short-term borrowings	168	58	32
Long-term debt	365	250	240
Securities loaned or sold under repurchase agreements	734	353	140
Fees from affiliates	1,213	866	957
Other	134	157	136
Other Noninterest Expense	409	389	395
MUSA and Various other Non-Bank Subsidiaries | Affiliated Entity
Related Party Transaction [Line Items]
Securities borrowed or purchased under resale agreements	66	41	26
Interest Income, Other	2	1	0
Interest Expense, Deposits	3	5	0
Commercial paper and other short-term borrowings	2	15	24
Long-term debt	171	114	69
Securities loaned or sold under repurchase agreements	18	6	2
Fees from affiliates	1,213	866	957
Other	1	13	(28)
Other Noninterest Expense	$ 115	$ 112	$ 95