Long-term Debt Other Long-term Debt (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2020		Dec. 31, 2019		Sep. 19, 2016	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]	$ 98,708		$ 98,576
Trust preferred subordinated debentures	[2]	60,254		60,250
Total long-term debt		158,962		158,826
5.50% Subordinated Notes
Debt Instruments [Abstract]
Subordinated notes, net of unamortized debt issuance costs:	[1],[3]	98,708		98,576
Unamortized Debt Issuance Expense		$ 1,300		1,400
Stated interest rate		5.50%				5.50%
Southside Statutory Trust III, net of unamortized debt issuance costs [Member]
Debt Instruments [Abstract]
Trust preferred subordinated debentures		$ 20,562	[2],[4]	20,558	[2],[4]						$ 20,619
Unamortized Debt Issuance Expense		57		61
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 $1.3 million at September 30, 2020 and $1.4 million at December 31, 2019.

[4] The unamortized debt issuance costs reflected in the carrying amount of the Southside Statutory Trust III junior subordinated debentures totaled $57,000 at September 30, 2020 and $61,000 at December 31, 2019.

[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.