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Long-term Debt LT Debt Interest Rates (Details) - USD ($) $ in Thousands | Nov. 14, 2025 | Aug. 10, 2007 | Aug. 08, 2007 | May 20, 2005 | Sep. 04, 2003 | Sep. 30, 2024 | Apr. 04, 2024 | Dec. 31, 2023 | Jun. 14, 2023 | Nov. 06, 2020 | Oct. 10, 2007
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Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt | [1] |  |  |  |  |  |  | $ 60,273 |  |  | $ 60,270 |  |  |  |
Southside Statutory Trust III |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt |  |  |  |  |  |  | $ 20,619 | 20,581 | [1],[2] |  | 20,578 | [1],[2] |  |  |
Southside Statutory Trust III | Three-Month Secured Overnight Financing Rate (SOFR) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate |  |  |  |  |  |  | 3.20% |  |  |  |  |  |  |  |
Southside Statutory Trust IV |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt |  |  |  | $ 23,196 |  |  |  | 23,196 | [1] |  | 23,196 | [1] |  |  |
Southside Statutory Trust IV | Three-Month Secured Overnight Financing Rate (SOFR) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate |  |  |  | 1.56% |  |  |  |  |  |  |  |  |  |  |
Southside Statutory Trust V |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt |  |  | $ 12,887 |  |  |  |  | 12,887 | [1] |  | 12,887 | [1] |  |  |
Southside Statutory Trust V | Three-Month Secured Overnight Financing Rate (SOFR) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate |  |  | 2.51% |  |  |  |  |  |  |  |  |  |  |  |
Magnolia Trust Company I |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt |  |  |  |  | $ 3,609 | [3] |  | $ 3,609 | [1] |  | $ 3,609 | [1] |  |  | $ 3,600
Magnolia Trust Company I | Three-Month Secured Overnight Financing Rate (SOFR) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate | [3] |  |  |  | 2.06% |  |  |  |  |  |  |  |  |  |
3.875% Subordinated Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt face amount | [4] |  |  |  |  |  |  |  |  |  |  |  |  | $ 100,000 |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.875% |
Debt instrument repurchase |  |  |  |  |  |  |  |  |  | $ 2,000 |  |  | $ 5,000 |  |
Forecast | 3.875% Subordinated Notes | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate |  | 3.66% |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | This debt consists of trust preferred securities that qualify under the risk-based capital guidelines as Tier 1 capital, subject to certain limitations.
[2] | The unamortized debt issuance costs reflected in the carrying amount of the Southside Statutory Trust III junior subordinated debentures totaled $38,000 at September 30, 2024 and $41,000 at December 31, 2023.
[3] | On October 10, 2007, as part of an acquisition we assumed $3.6 million of floating rate junior subordinated debentures issued in 2005 to Magnolia Trust Company I
[4] | On April 4, 2024 and June 14, 2023, the Company repurchased $2.0 million and $5.0 million, respectively, of the $100 million fixed-to-floating rate subordinated notes that mature on November 15, 2030.

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