EMPLOYEE BENEFITS - Pension Plan Asset Categories (Details) - USD ($) $ in Thousands		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Plan
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		$ 97,439	$ 90,419	$ 93,818	$ 84,911
Retirement Plan Acquired
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		3,871	3,613	$ 4,763	$ 4,070
Level 1 | Retirement Plan | Cash
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		3,495	1,593
Level 1 | Retirement Plan | U.S. large cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[1]	27,692	22,455
Level 1 | Retirement Plan | U.S. mid cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[2]	7,556	10,165
Level 1 | Retirement Plan | U.S. small cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[3]	15,004	12,934
Level 1 | Retirement Plan | International
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[4]	0	0
Level 1 | Retirement Plan | Corporate bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	0	0
Level 1 | Retirement Plan | International developed
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[6]	9,768	10,344
Level 1 | Retirement Plan | International emerging
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[2]	2,322	2,359
Level 1 | Retirement Plan | Real estate
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[7]	0	0
Level 1 | Retirement Plan | US Treasury and Government
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	262	0
Level 1 | Retirement Plan Acquired | Cash
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		0	0
Level 1 | Retirement Plan Acquired | U.S. large cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[1]	1,351	1,199
Level 1 | Retirement Plan Acquired | U.S. mid cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[2]	151	136
Level 1 | Retirement Plan Acquired | U.S. small cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[3]	70	64
Level 1 | Retirement Plan Acquired | International
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[4]	775	708
Level 1 | Retirement Plan Acquired | Corporate bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	1,277	1,226
Level 1 | Retirement Plan Acquired | International developed
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[6]	0	0
Level 1 | Retirement Plan Acquired | International emerging
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[2]	0	0
Level 1 | Retirement Plan Acquired | Real estate
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[7]	247	280
Level 1 | Retirement Plan Acquired | US Treasury and Government
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	0	0
Level 2 | Retirement Plan | Cash equivalents
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		9,949	16,689
Level 2 | Retirement Plan | Corporate bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	2,254	1,537
Level 2 | Retirement Plan | U.S. government agencies
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	5,437	4,677
Level 2 | Retirement Plan | Municipal bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	13,508	7,458
Level 2 | Retirement Plan | U.S. agency mortgage-backed securities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[8]	192	208
Level 2 | Retirement Plan Acquired | Cash equivalents
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		0	0
Level 2 | Retirement Plan Acquired | Corporate bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	0	0
Level 2 | Retirement Plan Acquired | U.S. government agencies
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	0	0
Level 2 | Retirement Plan Acquired | Municipal bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	0	0
Level 2 | Retirement Plan Acquired | U.S. agency mortgage-backed securities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[8]	$ 0	$ 0

[1] For the Retirement Plan, this category is comprised of broadly diversified “passive” and “active” mutual funds. The Acquired Retirement Plan assets in this category consist of pooled separate accounts invested in mutual funds and domestic stocks.

[2] For the Retirement Plan, this category is comprised of broadly diversified “active” mutual funds. The Acquired Retirement Plan assets in this category consist of pooled separate accounts invested in mutual funds and domestic stocks.

[3] For the Retirement Plan, this category is comprised of broadly diversified “passive” and “active” mutual funds and shares of Southside Bancshares stock. The Acquired Retirement Plan assets in this category consist of pooled separate accounts invested in mutual funds and domestic stocks.

[4] This category is comprised of pooled separate accounts invested in mutual funds and international stocks.

[5] For the Retirement Plan, this category is comprised of individual investment grade securities that are generally HTM. The Acquired Retirement Plan assets in this category consist of pooled separate accounts invested in mutual funds, investment grade and below investment grade bonds.

[6] This category is comprised of a broadly diversified “passive” and “active” mutual funds.

[7] This category is comprised of a pooled separate account invested in commercial real estate and includes mortgage loans which are backed by the associated properties.

[8] This category is comprised of individual securities that are generally not HTM.