Long-term Debt Other Long-term Debt (Details) - USD ($) $ in Thousands				9 Months Ended
		Sep. 30, 2021		Sep. 30, 2021		Dec. 31, 2020		Nov. 06, 2020	Sep. 19, 2016	Oct. 10, 2007	Aug. 10, 2007	Aug. 08, 2007	May 20, 2005	[5]	Sep. 04, 2003
Debt Instruments [Abstract]
Subordinated notes, net of unamortized debt issuance costs:	[1]	$ 98,500		$ 98,500		$ 197,251
Trust preferred subordinated debentures	[2]	60,259		60,259		60,255
Total long-term debt		158,759		158,759		257,506
5.50% Subordinated Notes
Debt Instruments [Abstract]
Subordinated notes, net of unamortized debt issuance costs:	[1],[3]	$ 0		0		98,754
Unamortized Debt Issuance Expense						$ 1,200
Stated interest rate						5.50%			5.50%
5.50% Subordinated Notes | London Interbank Offered Rate (LIBOR)
Debt Instruments [Abstract]
Basis on variable rate		4.297%
Southside Statutory Trust III, net of unamortized debt issuance costs
Debt Instruments [Abstract]
Trust preferred subordinated debentures		$ 20,567	[2],[4]	20,567	[2],[4]	$ 20,563	[2],[4]								$ 20,619
Unamortized Debt Issuance Expense		52		52		56
Southside Statutory Trust IV
Debt Instruments [Abstract]
Trust preferred subordinated debentures		23,196	[2]	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	[2]				$ 12,887
Magnolia Trust Company I
Debt Instruments [Abstract]
Trust preferred subordinated debentures		3,609	[2]	3,609	[2]	3,609	[2]			$ 3,600			$ 3,609
3.875% Subordinated Notes
Debt Instruments [Abstract]
Subordinated notes, net of unamortized debt issuance costs:	[1],[6]	98,500		98,500		98,497
Unamortized Debt Issuance Expense		$ 1,500		$ 1,500		$ 1,500
Stated interest rate		3.875%		3.875%				3.875%
Minimum
Debt Instruments [Abstract]
Subordinated notes, 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 $1.2 million at December 31, 2020.

[4] The unamortized debt issuance costs reflected in the carrying amount of the Southside Statutory Trust III junior subordinated debentures totaled $52,000 at September 30, 2021 and $56,000 at December 31, 2020.

[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.

[6] The unamortized discount and debt issuance costs reflected in the carrying amount of the subordinated notes totaled approximately $1.5 million at September 30, 2021 and December 31, 2020.