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Investment Securities - Narrative (Details) | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Jun. 30, 2025 USD ($) | Dec. 31, 2024 USD ($) security | Sep. 30, 2025 USD ($) security | Sep. 30, 2023 USD ($) | Jul. 07, 2023 USD ($)
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Held-to-maturity, allowance for credit loss |  | $ 0 | $ 0 |  |
Number of investment securities in unrealized loss position | security |  | 269 | 172 |  |
Number of investment securities in unrealized loss position longer than 12 months | security |  |  | 145 |  |
Number of investment securities in unrealized loss position less than 12 months | security |  |  | 27 |  |
Debt securities, available-for-sale unrealized loss position, allowance for credit loss |  | $ 0 | $ 0 |  |
Interest rate swap | Fair value hedge | Designated as hedging instrument |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Derivative, notional amount |  |  |  | $ 101,800,000 | $ 101,800,000
Loss on derivative contract termination |  | $ 0 |  |  |
Debt securities, available-for-sale, sold at par value | $ 69,100,000 |  |  |  |

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