Long-term Debt Other Long-term Obligations (Details) - USD ($)		9 Months Ended
		Sep. 30, 2019		Dec. 31, 2018		Oct. 10, 2007	Aug. 10, 2007	Aug. 08, 2007
Debt Instruments [Abstract]
Subordinated notes, net of unamortized debt issuance costs:	[1]	$ 98,532,000		$ 98,407,000
Trust preferred subordinated debentures	[2]	60,249,000		60,246,000
Total long-term debt		158,781,000		158,653,000
5.50% Subordinated Notes
Debt Instruments [Abstract]
Subordinated notes, net of unamortized debt issuance costs:	[1],[3]	98,532,000		98,407,000
Unamortized Debt Issuance Expense		1,500,000		1,600,000
Southside Statutory Trust III, net of unamortized debt issuance costs [Member]
Debt Instruments [Abstract]
Trust preferred subordinated debentures	[2],[4]	20,557,000		20,554,000
Unamortized Debt Issuance Expense		62,000		65,000
Southside Statutory Trust IV
Debt Instruments [Abstract]
Trust preferred subordinated debentures		23,196,000	[2]	23,196,000	[2]			$ 23,196,000
Southside Statutory Trust V
Debt Instruments [Abstract]
Trust preferred subordinated debentures		12,887,000	[2]	12,887,000	[2]		$ 12,887,000
Magnolia Trust Company I
Debt Instruments [Abstract]
Trust preferred subordinated debentures		$ 3,609,000	[2]	$ 3,609,000	[2]	$ 3,609,000
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 $1.5 million at September 30, 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 $62,000 at September 30, 2019 and $65,000 at December 31, 2018.