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Investment Securities - Additional Information (Details) - USD ($) |  |  |  | 3 Months Ended | 6 Months Ended
--- | --- | --- | --- | --- | ---
Jun. 30, 2025 | May 18, 2025 | Dec. 31, 2024 | Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |
Accrued interest receivable | $ 304,000,000 |  | $ 262,000,000 |  |  |  |
Proceeds from sales of securities related to the Transaction |  |  |  | $ 9,700,000,000 | $ 0 | $ 9,696,000,000 | $ 0
Investment securities | 38,500,000,000 |  | 39,400,000,000 | 38,500,000,000 |  | 38,500,000,000 |
Securities received as collateral | $ 7,000,000 |  | $ 18,000,000 | $ 7,000,000 |  | $ 7,000,000 |
Discover |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |
Business combination, recognized asset acquired, investment in debt and equity securities, current |  | $ 14,100,000,000 |  |  |  |  |
Business combination, recognized asset acquired, acquired debt securities sold |  | $ (9,700,000,000) |  |  |  |  |
Investment securities available for sale | Government Contracts Concentration Risk | US Treasury and Agency securities |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |
Percentage of portfolio | 97.00% |  | 96.00% |  |  |  |

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