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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
Mar. 31, 2024
Dec. 31, 2023
Jun. 14, 2023
Nov. 06, 2020
Oct. 10, 2007
Debt Instrument [Line Items]                    
Other long-term debt [1]           $ 60,271 $ 60,270      
Southside Statutory Trust III                    
Debt Instrument [Line Items]                    
Other long-term debt         $ 20,619 20,579 [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               $ 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 $40,000 at March 31, 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 June 14, 2023, the Company repurchased $5.0 million of the $100 million fixed-to-floating rate subordinated notes that mature on November 15, 2030.