EMPLOYEE BENEFITS - Pension Plan Asset Categories (Details) - USD ($) $ in Thousands		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Pension Plan
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		$ 90,419	$ 93,818	$ 84,911	$ 91,233
Defined Benefit Pension Plan Acquired
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		3,613	4,763	$ 4,070	$ 4,031
Level 1 | Defined Benefit Pension Plan | Cash
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		1,593	791
Level 1 | Defined Benefit Pension Plan | U.S. large cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[1]	22,455	11,482
Level 1 | Defined Benefit Pension Plan | U.S. mid cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[2]	10,165	11,081
Level 1 | Defined Benefit Pension Plan | U.S. small cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[3]	12,934	17,846
Level 1 | Defined Benefit Pension Plan | International
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[4]	0	0
Level 1 | Defined Benefit Pension Plan | Corporate bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	0	0
Level 1 | Defined Benefit Pension Plan | International developed
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[6]	10,344	8,771
Level 1 | Defined Benefit Pension Plan | International emerging
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[2]	2,359	4,383
Level 1 | Defined Benefit Pension Plan | Real estate
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[7]	0	0
Level 1 | Defined Benefit Pension Plan | Balanced asset allocation
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[8]	0	0
Level 1 | Defined Benefit Pension Plan Acquired | Cash
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		0	0
Level 1 | Defined Benefit Pension Plan Acquired | U.S. large cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[1]	1,199	1,548
Level 1 | Defined Benefit Pension Plan Acquired | U.S. mid cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[2]	136	183
Level 1 | Defined Benefit Pension Plan Acquired | U.S. small cap equities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[3]	64	87
Level 1 | Defined Benefit Pension Plan Acquired | International
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[4]	708	992
Level 1 | Defined Benefit Pension Plan Acquired | Corporate bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	1,226	1,580
Level 1 | Defined Benefit Pension Plan Acquired | International developed
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[6]	0	0
Level 1 | Defined Benefit Pension Plan Acquired | International emerging
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[2]	0	0
Level 1 | Defined Benefit Pension Plan Acquired | Real estate
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[7]	280	278
Level 1 | Defined Benefit Pension Plan Acquired | Balanced asset allocation
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[8]	0	95
Level 2 | Defined Benefit Pension Plan | Cash equivalents
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		16,689	16,007
Level 2 | Defined Benefit Pension Plan | Corporate bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	1,537	965
Level 2 | Defined Benefit Pension Plan | U.S. government agencies
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	4,677	17,732
Level 2 | Defined Benefit Pension Plan | Municipal bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	7,458	4,508
Level 2 | Defined Benefit Pension Plan | U.S. agency mortgage-backed securities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[9]	208	252
Level 2 | Defined Benefit Pension Plan Acquired | Cash equivalents
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets		0	0
Level 2 | Defined Benefit Pension Plan Acquired | Corporate bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	0	0
Level 2 | Defined Benefit Pension 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 | Defined Benefit Pension Plan Acquired | Municipal bonds
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[5]	0	0
Level 2 | Defined Benefit Pension Plan Acquired | U.S. agency mortgage-backed securities
Defined Benefit Plan, Categories of Plan Assets [Abstract]
Total fair value of plan assets	[9]	$ 0	$ 0

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

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

[3] For the Plan, this category is comprised of broadly diversified “passive” mutual funds and shares of Southside Bancshares stock. The Acquired 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 Plan, this category is comprised of individual investment grade securities that are generally HTM. The Acquired 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 a pooled separate account invested in a single mutual fund invested in a combination of fixed income and equity investment options.

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