LONG-TERM DEBT - Other Long-term Obligations (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018		Oct. 10, 2007	[5]	Aug. 10, 2007	Aug. 08, 2007	Sep. 04, 2003
Debt Instruments [Abstract]
Subordinated notes, net of unamortized debt issuance costs	[1]	$ 98,576		$ 98,407
Trust preferred subordinated debentures	[2]	60,250		60,246
Total long-term obligations		158,826		158,653
5.50% Subordinated Notes
Debt Instruments [Abstract]
Subordinated notes, net of unamortized debt issuance costs	[1],[3]	98,576		98,407
Unamortized debt issuance expense		1,400		1,600
Southside Statutory Trust III, net of unamortized debt issuance costs
Debt Instruments [Abstract]
Trust preferred subordinated debentures		20,558	[2],[4]	20,554	[2],[4]					$ 20,619
Unamortized debt issuance expense		61		65
Southside Statutory Trust IV
Debt Instruments [Abstract]
Trust preferred subordinated debentures		23,196	[2]	23,196	[2]				$ 23,196
Southside Statutory Trust V
Debt Instruments [Abstract]
Trust preferred subordinated debentures		12,887	[2]	12,887	[2]			$ 12,887
Magnolia Trust Company I
Debt Instruments [Abstract]
Trust preferred subordinated debentures		$ 3,609	[2]	$ 3,609	[2]	$ 3,609
Minimum | 5.50% Subordinated Notes
Debt Instruments [Abstract]
Long-term debt, remaining maturity, greater than		1 year

[1] This debt consists of subordinated notes with a remaining maturity greater than one year that qualify under the risk-based capital guidelines as Tier 2 capital, subject to certain limitations.

[2] This debt consists of trust preferred securities that qualify under the risk-based capital guidelines as Tier 1 capital, subject to certain limitations.

[3] The unamortized discount and debt issuance costs reflected in the carrying amount of the subordinated notes totaled approximately $1.4 million at December 31, 2019 and $1.6 million at December 31, 2018.

[4] The unamortized debt issuance costs reflected in the carrying amount of the Southside Statutory Trust III junior subordinated debentures totaled $61,000 at December 31, 2019 and $65,000 at December 31, 2018.

[5] 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.