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SUBORDINATED DEBENTURES (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2015 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2017 | Feb. 01, 2021 | Dec. 31, 2020
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Fixed-to-floating rate subordinated debentures | $ 80.0 |  |  |  |  |  |  |
Subordinated notes payable, net |  | $ 197.1 |  | $ 197.1 |  |  |  | $ 114.1
Interest expense related to the subordinated debt |  | $ 2.2 | $ 1.3 | $ 6.3 | $ 4.0 |  |  |
Subordinated Debentures |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Fixed-to-floating rate subordinated debentures |  |  |  |  |  | $ 115.0 | $ 115.0 |
Fixed annual interest rate |  |  |  |  |  | 4.50% | 4.50% |
Basis points |  |  |  |  |  | 2.66% |  |
Callable Notes After Five Years | Subordinated Debentures |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Fixed-to-floating rate subordinated debentures | $ 40.0 |  |  |  |  |  |  |
Debt Instrument, Term | 5 years |  |  |  |  |  |  |
Fixed annual interest rate | 0.0525% |  |  |  |  |  |  |
Debt instrument variable rate description | three-month LIBOR |  |  |  |  |  |  |
Basis points | 3.60% |  |  |  |  |  |  |
Callable Notes After Ten Years | Subordinated Debentures |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |
Fixed-to-floating rate subordinated debentures | $ 40.0 |  |  |  |  |  |  |
Debt Instrument, Term | 10 years |  |  |  |  |  |  |
Fixed annual interest rate | 0.0575% |  |  |  |  |  |  |
Debt instrument variable rate description | three-month LIBOR |  |  |  |  |  |  |
Basis points | 3.45% |  |  |  |  |  |  |

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