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Long-term Debt LT Debt Interest Rates (Details) - USD ($) | Nov. 14, 2025 | Aug. 10, 2007 | Aug. 08, 2007 | May 20, 2005 | Sep. 04, 2003 | Mar. 31, 2022 | Dec. 31, 2021 | Nov. 06, 2020 | Oct. 10, 2007
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Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt face amount |  |  |  |  |  |  |  | $ 575,000,000 |  |  |  |  |
Other long-term debt | [1] |  |  |  |  |  |  | 60,261,000 |  | $ 60,260,000 |  |  |
Southside Statutory Trust III |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt |  |  |  |  |  |  | $ 20,619,000 | 20,569,000 | [1],[2] | 20,568,000 | [1],[2] |  |
Southside Statutory Trust III | Three-Month London Interbank Offered Rate (LIBOR) |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate |  |  |  |  |  |  | 2.94% |  |  |  |  |  |
Southside Statutory Trust IV |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt |  |  |  | $ 23,196,000 |  |  |  | 23,196,000 | [1] | 23,196,000 | [1] |  |
Southside Statutory Trust IV | Three-Month London Interbank Offered Rate (LIBOR) |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate |  |  |  | 1.30% |  |  |  |  |  |  |  |  |
Southside Statutory Trust V |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt |  |  | $ 12,887,000 |  |  |  |  | 12,887,000 | [1] | 12,887,000 | [1] |  |
Southside Statutory Trust V | Three-Month London Interbank Offered Rate (LIBOR) |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate |  |  | 2.25% |  |  |  |  |  |  |  |  |  |
Magnolia Trust Company I |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Other long-term debt |  |  |  |  | $ 3,609,000 | [3] |  | $ 3,609,000 | [1] | $ 3,609,000 | [1] |  | $ 3,600,000
Magnolia Trust Company I | Three-Month London Interbank Offered Rate (LIBOR) |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Basis on variable rate |  |  |  |  | 1.80% |  |  |  |  |  |  |  |
3.875% Subordinated Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt face amount |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000 |
Stated interest rate |  |  |  |  |  |  |  | 3.875% |  |  |  | 3.875% |
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 $50,000 at March 31, 2022 and $51,000 at December 31, 2021.
[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.

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