LONG-TERM DEBT - Other Long-term Debt (Details) - USD ($) $ in Thousands |
12 Months Ended | ||||||||||||||||||||
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Dec. 31, 2020 |
Dec. 31, 2019 |
Oct. 10, 2007 |
[6] | Aug. 10, 2007 |
Aug. 08, 2007 |
Sep. 04, 2003 |
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Debt Instruments [Abstract] | |||||||||||||||||||||
Subordinated notes, net of unamortized debt issuance costs | [1] | $ 197,251 | $ 98,576 | ||||||||||||||||||
Trust preferred subordinated debentures | [2] | 60,255 | 60,250 | ||||||||||||||||||
Total long-term obligations | 257,506 | 158,826 | |||||||||||||||||||
3.875% Subordinated Notes | |||||||||||||||||||||
Debt Instruments [Abstract] | |||||||||||||||||||||
Subordinated notes, net of unamortized debt issuance costs | [1],[3] | 98,497 | 0 | ||||||||||||||||||
Unamortized debt issuance expense | $ 1,500 | ||||||||||||||||||||
Stated interest rate | 3.875% | ||||||||||||||||||||
5.50% Subordinated Notes | |||||||||||||||||||||
Debt Instruments [Abstract] | |||||||||||||||||||||
Subordinated notes, net of unamortized debt issuance costs | [1],[4] | $ 98,754 | 98,576 | ||||||||||||||||||
Unamortized debt issuance expense | $ 1,200 | 1,400 | |||||||||||||||||||
Stated interest rate | 5.50% | ||||||||||||||||||||
Southside Statutory Trust III, net of unamortized debt issuance costs | |||||||||||||||||||||
Debt Instruments [Abstract] | |||||||||||||||||||||
Trust preferred subordinated debentures | $ 20,563 | [2],[5] | 20,558 | [2],[5] | $ 20,619 | ||||||||||||||||
Unamortized debt issuance expense | 56 | 61 | |||||||||||||||||||
Southside Statutory Trust IV | |||||||||||||||||||||
Debt Instruments [Abstract] | |||||||||||||||||||||
Trust preferred subordinated debentures | 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 | ||||||||||||||||
Magnolia Trust Company I | |||||||||||||||||||||
Debt Instruments [Abstract] | |||||||||||||||||||||
Trust preferred subordinated debentures | $ 3,609 | [2] | $ 3,609 | [2] | $ 3,609 | ||||||||||||||||
Minimum | 5.50% Subordinated Notes | |||||||||||||||||||||
Debt Instruments [Abstract] | |||||||||||||||||||||
Long-term debt, remaining maturity, greater than | 1 year | ||||||||||||||||||||
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