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INVESTMENT SECURITIES AND OTHER SECURITIES - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2025 USD ($) | Dec. 31, 2024 USD ($) | Sep. 30, 2025 USD ($) security
Investment Securities [Line Items] |  |  |
Number of municipal bonds classified as AFS debt securities | security |  |  | 1
AFS debt security charge-off |  | $ 2 |
Allowance related to investment securities, available for sale securities |  | 0 | $ 0
Recovery of previously written-off available-for-sale municipal security | $ 3 |  |
Allowance related to investment securities |  | 0 | 0
Asset Pledged as Collateral |  |  |
Investment Securities [Line Items] |  |  |
Investment securities pledged |  | $ 37,700 | $ 30,800

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