Long-term Debt LT Debt Interest Rates (Details) - USD ($)		Nov. 14, 2025	Sep. 30, 2021	Aug. 10, 2007	Aug. 08, 2007	May 20, 2005		Sep. 04, 2003	Jun. 30, 2021		Dec. 31, 2020		Nov. 06, 2020	Sep. 19, 2016	Oct. 10, 2007
Debt Instrument [Line Items]
Debt face amount									$ 605,000,000.0
Other long-term debt	[1]								$ 60,258,000		$ 60,255,000
5.50% Subordinated Notes
Debt Instrument [Line Items]
Debt face amount														$ 100,000,000.0
Stated interest rate									5.50%					5.50%
Southside Statutory Trust III
Debt Instrument [Line Items]
Other long-term debt								$ 20,619,000	$ 20,566,000	[1],[2]	20,563,000	[1],[2]
Southside Statutory Trust III | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis on variable rate								2.94%
Southside Statutory Trust IV
Debt Instrument [Line Items]
Other long-term debt					$ 23,196,000				23,196,000	[1]	23,196,000	[1]
Southside Statutory Trust IV | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis on variable rate					1.30%
Southside Statutory Trust V
Debt Instrument [Line Items]
Other long-term debt				$ 12,887,000					12,887,000	[1]	12,887,000	[1]
Southside Statutory Trust V | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis on variable rate				2.25%
Magnolia Trust Company I
Debt Instrument [Line Items]
Other long-term debt						$ 3,609,000	[3]		$ 3,609,000	[1]	$ 3,609,000	[1]			$ 3,600,000
Magnolia Trust Company I | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis on variable rate						1.80%
3.875% Subordinated Notes
Debt Instrument [Line Items]
Debt face amount													$ 100,000,000.0
Stated interest rate									3.875%				3.875%
Forecast | 5.50% Subordinated Notes | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis on variable rate			4.297%
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 $53,000 at June 30, 2021 and $56,000 at December 31, 2020.

[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.