Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ExtendedMaturityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LoanRestructuringModificationAxis=sbsi_CombinationofConcessionsGrantedinLoanRestructuringModificationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Allowance for Loan Losses - Troubled Debt Restructurings (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) contract	Jun. 30, 2020 USD ($) contract	Jun. 30, 2021 USD ($) contract	Jun. 30, 2020 USD ($) contract
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| contract	2	2	5	4
Total Modifications	$ 106	$ 148	$ 250	$ 209
1-4 Family Residential Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| contract			2
Total Modifications			$ 128
Commercial Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| contract				1
Total Modifications				$ 59
Commercial loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| contract	2	2	3	3
Total Modifications	$ 106	$ 148	$ 122	$ 150
Extended Amortization Period
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications	0	0	0	0
Extended Amortization Period \| 1-4 Family Residential Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications			0
Extended Amortization Period \| Commercial Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications				0
Extended Amortization Period \| Commercial loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications	0	0	0	0
Interest Rate Reduction
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications	0	0	0	0
Interest Rate Reduction \| 1-4 Family Residential Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications			0
Interest Rate Reduction \| Commercial Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications				0
Interest Rate Reduction \| Commercial loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications	0	0	0	0
Combination
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications	106	148	250	209
Combination \| 1-4 Family Residential Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications			128
Combination \| Commercial Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications				59
Combination \| Commercial loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Total Modifications	$ 106	$ 148	$ 122	$ 150