Long-term Debt LT Debt Interest Rates (Details) - USD ($) $ in Thousands		Nov. 14, 2025	Sep. 30, 2021	Nov. 06, 2020	Sep. 19, 2016	Oct. 10, 2007		Aug. 10, 2007	Aug. 08, 2007	Sep. 04, 2003	Mar. 31, 2021		Dec. 31, 2020
Debt Instrument [Line Items]
Other long-term debt	[1]										$ 60,256		$ 60,255
5.50% Subordinated Notes
Debt Instrument [Line Items]
Amount issued					$ 100,000
Stated interest rate					5.50%						5.50%
Southside Statutory Trust III
Debt Instrument [Line Items]
Other long-term debt										$ 20,619	$ 20,564	[1],[2]	20,563	[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		23,196	[1]	23,196	[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			12,887	[1]	12,887	[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	[3]				$ 3,609	[1]	$ 3,609	[1]
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]
Amount issued				$ 100,000
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 $55,000 at March 31, 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.