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Long-term Debt LT Debt Interest Rates (Details) - USD ($)
$ in Thousands
Aug. 15, 2030
Nov. 14, 2025
Aug. 10, 2007
Aug. 08, 2007
May 20, 2005
Sep. 04, 2003
Sep. 30, 2025
Aug. 14, 2025
Dec. 31, 2024
Apr. 04, 2024
Jun. 14, 2023
Nov. 06, 2020
Oct. 10, 2007
Debt Instrument [Line Items]                          
Other long-term debt [1]             $ 60,278   $ 60,274        
Southside Statutory Trust III                          
Debt Instrument [Line Items]                          
Other long-term debt           $ 20,619 20,586 [1],[2]   20,582 [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    
7.0% Subordinated Notes Due 2035                          
Debt Instrument [Line Items]                          
Debt face amount               $ 150,000          
Stated interest rate               7.00%          
Forecast | 3.875% Subordinated Notes | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate                          
Debt Instrument [Line Items]                          
Basis on variable rate   3.66%                      
Forecast | 7.0% Subordinated Notes Due 2035 | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate                          
Debt Instrument [Line Items]                          
Basis on variable rate 3.57%                        
[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 $33,000 at September 30, 2025 and $37,000 at December 31, 2024.
[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.