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Long-term Debt Other Long-term Debt (Details) - USD ($) $ in Thousands | 9 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2023 | Dec. 31, 2022 | Nov. 06, 2020 | Oct. 10, 2007 | Aug. 10, 2007 | Aug. 08, 2007 | May 20, 2005 | [4] | Sep. 04, 2003
Debt Instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |
Subordinated notes, net of unamortized debt issuance costs: | [1] | $ 93,838 |  | $ 98,674 |  |  |  |  |  |  |
Trust preferred subordinated debentures | [2] | 60,269 |  | 60,265 |  |  |  |  |  |  |
Total long-term debt |  | 154,107 |  | 158,939 |  |  |  |  |  |  |
Southside Statutory Trust III, net of unamortized debt issuance costs |  |  |  |  |  |  |  |  |  |  |
Debt Instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |
Trust preferred subordinated debentures |  | 20,577 | [2],[3] | 20,573 | [2],[3] |  |  |  |  |  | $ 20,619
Unamortized Debt Issuance Expense |  | 42 |  | 46 |  |  |  |  |  |  |
Southside Statutory Trust IV |  |  |  |  |  |  |  |  |  |  |
Debt Instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |
Trust preferred subordinated debentures |  | 23,196 | [2] | 23,196 | [2] |  |  |  | $ 23,196 |  |
Southside Statutory Trust V |  |  |  |  |  |  |  |  |  |  |
Debt Instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |
Trust preferred subordinated debentures |  | 12,887 | [2] | 12,887 | [2] |  |  | $ 12,887 |  |  |
Magnolia Trust Company I |  |  |  |  |  |  |  |  |  |  |
Debt Instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |
Trust preferred subordinated debentures |  | 3,609 | [2] | 3,609 | [2] |  | $ 3,600 |  |  | $ 3,609 |
3.875% Subordinated Notes |  |  |  |  |  |  |  |  |  |  |
Debt Instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |
Subordinated notes, net of unamortized debt issuance costs: | [1],[5] | 93,838 |  | 98,674 |  |  |  |  |  |  |
Unamortized Debt Issuance Expense |  | $ 1,200 |  | $ 1,300 |  |  |  |  |  |  |
Stated interest rate |  | 3.875% |  |  |  | 3.875% |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |
Debt Instruments [Abstract] |  |  |  |  |  |  |  |  |  |  |
Subordinated notes, remaining maturity, greater than |  | 1 year |  |  |  |  |  |  |  |  |
[1] | This debt consists of subordinated notes with a remaining maturity greater than one year that qualify under the risk-based capital guidelines as Tier 2 capital, subject to certain limitations.
[2] | This debt consists of trust preferred securities that qualify under the risk-based capital guidelines as Tier 1 capital, subject to certain limitations.
[3] | The unamortized debt issuance costs reflected in the carrying amount of the Southside Statutory Trust III junior subordinated debentures totaled $42,000 at September 30, 2023 and $46,000 at December 31, 2022.
[4] | 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
[5] | The unamortized discount and debt issuance costs reflected in the carrying amount of the subordinated notes totaled approximately $1.2 million at September 30, 2023 and $1.3 million at December 31, 2022.

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