LONG-TERM DEBT - LT Debt Interest Rates (Details) - USD ($)									9 Months Ended
		Nov. 14, 2025	Sep. 30, 2021	Aug. 10, 2007	Aug. 08, 2007	May 20, 2005		Sep. 04, 2003	Sep. 30, 2021	Dec. 31, 2021		Dec. 31, 2020		Nov. 06, 2020	Sep. 19, 2016	Oct. 10, 2007
Debt Instrument [Line Items]
Debt face amount										$ 605,000,000
Other long-term debt	[1]									60,260,000		$ 60,255,000
3.875% Subordinated Notes
Debt Instrument [Line Items]
Debt face amount														$ 100,000,000
Stated interest rate														3.875%
5.50% Subordinated Notes
Debt Instrument [Line Items]
Debt face amount															$ 100,000,000
Stated interest rate												5.50%			5.50%
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed									100.00%
5.50% Subordinated Notes | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate			4.297%
Southside Statutory Trust III
Debt Instrument [Line Items]
Other long-term debt								$ 20,619,000		20,568,000	[1],[2]	$ 20,563,000	[1],[2]
Southside Statutory Trust III | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread 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 spread 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 spread 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 spread on variable rate	[3]					1.80%
Forecast [Member] | 3.875% Subordinated Notes | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Debt Instrument [Line Items]
Basis spread 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 $51,000 at December 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.