LONG-TERM DEBT - LT Debt Interest Rates (Details) - USD ($) $ in Thousands		Sep. 29, 2021	Sep. 19, 2016	Oct. 10, 2007		Aug. 10, 2007	Aug. 08, 2007	Sep. 04, 2003	Dec. 31, 2018		Dec. 31, 2017
Debt Instrument [Line Items]
Other long-term debt	[1]								$ 60,246		$ 60,241
5.50% Subordinated Notes
Debt Instrument [Line Items]
Amount issued			$ 100,000
Stated interest rate			5.50%
Southside Statutory Trust III
Debt Instrument [Line Items]
Amount issued								$ 20,619
Other long-term debt	[1],[2]								20,554		20,549
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		23,196	[1]	23,196	[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			12,887	[1]	12,887	[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	[3]				$ 3,609	[1]	$ 3,609	[1]
Magnolia Trust Company I | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate	[3]			1.80%
Scenario, Forecast [Member] | 5.50% Subordinated Notes | Three-Month London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate		4.297%

[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 $65,000 at December 31, 2018 and $70,000 at December 31, 2017.

[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.