Related Party Transactions - Narrative (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Related Party Transaction [Line Items]
Gain on sale	$ 1,489,000,000	$ 1,439,000,000	$ 1,213,000,000
Available-for-sale Securities	18,259,000,000	17,789,000,000
Increase in retained earnings	(23,000,000)	(57,000,000)	4,000,000
Liabilities	150,567,000,000	154,443,000,000
Derivative, Notional Amount	220,226,000,000	241,444,000,000
MUSA and Various other Non-Bank Subsidiaries
Related Party Transaction [Line Items]
Derivative, Notional Amount	4,600,000,000	4,000,000,000.0
Unrealized gains	14,000,000	68,000,000
Unsecured Debt [Member] \| MUSA and Various other Non-Bank Subsidiaries
Related Party Transaction [Line Items]
Unsecured revolving credit facility with BTMU	1,600,000,000
Retained Earnings
Related Party Transaction [Line Items]
Increase in retained earnings	(4,000,000)	(5,000,000)	0
Affiliated Entity [Member] \| MUSA and Various other Non-Bank Subsidiaries
Related Party Transaction [Line Items]
Gain on sale	1,489,000,000	$ 1,439,000,000	$ 1,213,000,000
Securities sold under agreements to repurchase	0
Sale Of Loans \| Affiliated Entity [Member]
Related Party Transaction [Line Items]
Proceeds from sale of loan	1,500,000,000
Gain on sale	3,000,000
Purchase of loans	$ 56,000,000

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- Definition

Securities Sold under Agreements to Repurchase, Guaranteed Balance

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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No definition available.

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- Definition

Nominal or face amount used to calculate payment on derivative.

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Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of transactions with related party during the financial reporting period.

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Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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This element represents movements included in the statement of changes in stockholders' equity which are not separately disclosed or provided for elsewhere in the taxonomy.

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