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Long-term Debt Other Long-term Debt (Details) - USD ($)
$ in Thousands
 9 Months Ended
Sep. 30, 2025
Aug. 14, 2025
Dec. 31, 2024
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] $ 239,601   $ 92,042            
Trust preferred subordinated debentures [2] 60,278   60,274            
Total long-term debt 299,879   152,316            
Southside Statutory Trust III, net of unamortized debt issuance costs                  
Debt Instruments [Abstract]                  
Trust preferred subordinated debentures 20,586 [2],[3]   20,582 [2],[3]           $ 20,619
Unamortized Debt Issuance Expense 33   37            
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] 92,152   92,042            
Unamortized Debt Issuance Expense 848   958            
Stated interest rate       3.875%          
7.0% Subordinated Notes Due 2035                  
Debt Instruments [Abstract]                  
Subordinated notes, net of unamortized debt issuance costs: [1],[6] $ 147,449   $ 0            
Stated interest rate   7.00%              
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 $33,000 at September 30, 2025 and $37,000 at December 31, 2024.
[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 $848,000 at September 30, 2025 and $958,000 at December 31, 2024.
[6] The unamortized discount and debt issuance costs reflected in the carrying amount of the subordinated notes totaled approximately $2.6 million at September 30, 2025