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Long-term Obligations - Long-term Obligations (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Rate
Dec. 31, 2014
Debt Instruments [Abstract]    
FHLB advances $ 549,545us-gaap_FederalHomeLoanBankAdvancesLongTerm [1] $ 600,052us-gaap_FederalHomeLoanBankAdvancesLongTerm [1]
Long-term debt 60,311us-gaap_OtherLongTermDebt [2] 60,311us-gaap_OtherLongTermDebt [2]
Total long-term obligations 609,856us-gaap_LongTermDebt 660,363us-gaap_LongTermDebt
Federal Home Loan Bank Advances    
Advances from Federal Home Loan Banks [Abstract]    
Weighted average cost (in hundredths) 1.32%us-gaap_DebtWeightedAverageInterestRate
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember		  
Maturity date range, earliest Jul. 25, 2016  
Maturity date range, latest Jul. 03, 2028  
Southside Statutory Trust III    
Debt Instruments [Abstract]    
Long-term debt 20,619us-gaap_OtherLongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustIiiMember		 [2],[3] 20,619us-gaap_OtherLongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustIiiMember		 [2]
Long-term Debt, Other Disclosures [Abstract]    
Maturity date 2033  
Adjusted rate of debt 3.2154%us-gaap_LongTermDebtPercentageBearingVariableInterestRate
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustIiiMember		  
Date of interest rate adjustment June 29, 2015  
Debt instrument, description of variable rate basis three-month LIBOR plus 294 basis points  
Southside Statutory Trust IV    
Debt Instruments [Abstract]    
Long-term debt 23,196us-gaap_OtherLongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustIvMember		 [2],[4] 23,196us-gaap_OtherLongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustIvMember		 [2]
Long-term Debt, Other Disclosures [Abstract]    
Maturity date 2037  
Adjusted rate of debt 1.5546%us-gaap_LongTermDebtPercentageBearingVariableInterestRate
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustIvMember		  
Date of interest rate adjustment April 29, 2015  
Debt instrument, description of variable rate basis three-month LIBOR plus 130 basis points  
Southside Statutory Trust V    
Debt Instruments [Abstract]    
Long-term debt 12,887us-gaap_OtherLongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustVMember		 [2],[5] 12,887us-gaap_OtherLongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustVMember		 [2]
Long-term Debt, Other Disclosures [Abstract]    
Maturity date 2037  
Adjusted rate of debt 2.5206%us-gaap_LongTermDebtPercentageBearingVariableInterestRate
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustVMember		  
Date of interest rate adjustment June 14, 2015  
Debt instrument, description of variable rate basis three-month LIBOR plus 225 basis points  
Magnolia Trust Company I    
Debt Instruments [Abstract]    
Long-term debt $ 3,609us-gaap_OtherLongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= sbsi_MagnoliaTrustCompanyIMember		 [2],[6] $ 3,609us-gaap_OtherLongTermDebt
/ us-gaap_LongtermDebtTypeAxis
= sbsi_MagnoliaTrustCompanyIMember		 [2]
Long-term Debt, Other Disclosures [Abstract]    
Maturity date 2035  
Adjusted rate of debt 2.0615%us-gaap_LongTermDebtPercentageBearingVariableInterestRate
/ us-gaap_LongtermDebtTypeAxis
= sbsi_MagnoliaTrustCompanyIMember		  
Date of interest rate adjustment May 25, 2015  
Debt instrument, description of variable rate basis three-month LIBOR plus 180 basis points  
LIBOR | Southside Statutory Trust III    
Long-term Debt, Other Disclosures [Abstract]    
Basis spread on variable rate 2.94%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustIiiMember
/ us-gaap_VariableRateAxis
= sbsi_LiborMember		  
LIBOR | Southside Statutory Trust IV    
Long-term Debt, Other Disclosures [Abstract]    
Basis spread on variable rate 1.30%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustIvMember
/ us-gaap_VariableRateAxis
= sbsi_LiborMember		  
LIBOR | Southside Statutory Trust V    
Long-term Debt, Other Disclosures [Abstract]    
Basis spread on variable rate 2.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= sbsi_SouthsideStatutoryTrustVMember
/ us-gaap_VariableRateAxis
= sbsi_LiborMember		  
LIBOR | Magnolia Trust Company I    
Long-term Debt, Other Disclosures [Abstract]    
Basis spread on variable rate 1.80%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= sbsi_MagnoliaTrustCompanyIMember
/ us-gaap_VariableRateAxis
= sbsi_LiborMember		  
[1] At March 31, 2015, the weighted average cost of these advances was 1.32%. Long-term FHLB Advances have maturities ranging from July 2016 through July 2028.
[2] This long-term debt consists of trust preferred securities that qualify under the risk-based capital guidelines as Tier 1 capital, subject to certain limitations.
[3] This debt carries an adjustable rate of 3.2154% through June 29, 2015 and adjusts quarterly thereafter at a rate equal to three-month LIBOR plus 294 basis points.
[4] This debt carries an adjustable rate of 1.5546% through April 29, 2015 and adjusts quarterly thereafter at a rate equal to three-month LIBOR plus 130 basis points.
[5] This debt carries an adjustable rate of 2.5206% through June 14, 2015 and adjusts quarterly thereafter at a rate equal to three-month LIBOR plus 225 basis points.
[6] This debt carries an adjustable rate of 2.0615% through May 25, 2015 and adjusts quarterly thereafter at a rate equal to three-month LIBOR plus 180 basis points.