Securities - Interest Income on Securities (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Investments, Debt Securities [Abstract]
U.S. Treasury	$ 1,436	$ 2,066	$ 3,003	$ 4,089
State and political subdivisions	11,659	13,888	23,319	28,157
Corporate bonds and other	1,593	1,765	3,210	3,528
MBS	13,040	11,084	26,563	21,203
Total interest income on securities	$ 27,728	$ 28,803	$ 56,095	$ 56,977

Investments, Debt Securities [Abstract]

No definition available.

Name:	sbsi_InvestmentsDebtSecuritiesAbstract
Namespace Prefix:	sbsi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of interest income and other income recognized during the period. Included in this element is interest derived from investments in debt securities, cash and cash equivalents, and other investments which reflect the time value of money or transactions in which the payments are for the use or forbearance of money and other income from ancillary business-related activities (that is, excluding major activities considered part of the normal operations of the business).

+ References

No definition available.

+ Details

Name:	us-gaap_InterestAndOtherIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of operating interest income, including amortization and accretion of premiums and discounts, on corporate debt securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

+ Details

Name:	us-gaap_InterestIncomeSecuritiesCorporateDebtOperating
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of operating interest income, including amortization and accretion of premiums and discounts, on securitized, pass-through debt securities collateralized by real estate mortgage loans.

+ References

+ Details

Name:	us-gaap_InterestIncomeSecuritiesMortgageBacked
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Interest income, including any amortization and accretion (as applicable) of discounts and premiums, derived from debt securities issued by municipal governments, local taxing districts and US state government agencies.

+ References

+ Details

Name:	us-gaap_InterestIncomeSecuritiesStateAndMunicipal
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Interest income on US treasury securities. US treasury securities are negotiable debt obligations of the US government, secured by its full faith and credit and issued at various schedules and maturities. The income from treasury securities is exempt from state and local, but not federal, taxes. There are three types of securities issued by the US treasury (bonds, bills, and notes), which are distinguished by the amount of time from the initial sale of the bond to maturity.

+ References

+ Details

Name:	us-gaap_InterestIncomeSecuritiesUSTreasury
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration