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EMPLOYEE BENEFITS - Stock Option Valuation Assumptions (Details) - Stock options - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Weighted-average grant date fair value per option (in dollars per share) $ 5.89 $ 6.78
Weighted-average assumptions:    
Risk-free interest rates 1.63% 2.81%
Expected dividend yield 3.50% 1.10%
Expected volatility factors of the market price of Southside Bancshares common stock 25.80% 25.41%
Expected option life (in years) 6 years 2 months 12 days 6 years 2 months 12 days