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BORROWINGS - Narrative (Details) |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 18, 2024 | Nov. 01, 2013 USD ($) investor | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($) | Dec. 18, 2019 USD ($)
Debt Instrument [Line Items] |  |  |  |  |
Required value of assets pledged as collateral as a percentage to outstanding advances (at least) |  |  | 145.00% |  |
Total available remaining borrowing capacity from FHLB |  |  | $ 664,900,000 |  |
Putable advances with the FHLB |  |  | 712,852,000 | $ 823,674,000 |
Debt face amount |  | $ 70,000,000 |  |  |
Private debt issuance, number of institutional investors | investor |  | 4 |  |  |
Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Fixed interest rate term |  | 10 years |  |  |
5.00% Senior Notes Due 2028 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Redemption price (percent) |  | 100.00% |  |  |
Putable Advances |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Putable advances with the FHLB |  |  | $ 210,000,000 |  |
Minimum |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest rate, minimum (as a percent) |  |  | 0.35% |  |
Maximum |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest rate, minimum (as a percent) |  |  | 4.94% |  |
Securities Sold Under Repurchase Agreements |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum amount of outstanding agreements |  |  | $ 242,200,000 | 218,900,000 |
Total of average agreements |  |  | 171,300,000 | 185,100,000 |
Debt outstanding |  |  | 157,280,000 |  |
Subordinated Debentures and Term Loans |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt outstanding |  |  | 158,644,000 | 151,300,000 |
Subordinated Debenture | 6.75% Subordinated Notes Due 2028 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt face amount |  | $ 65,000,000 |  |  |
Interest rate on notes (as a percent) |  | 6.75% |  |  |
Redemption price (percent) |  | 100.00% |  |  |
Subordinated Debenture | 6.75% Subordinated Notes Due 2028 | Notice of intent to redeem |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Principal redemption |  |  | $ 40,000,000 |  |
Subordinated Debenture | Acquired Subordinated Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt face amount |  |  |  |  | $ 30,000,000
Interest rate on notes (as a percent) |  |  |  |  | 4.75%
Subordinated Debenture | Three-Month SOFR | 6.75% Subordinated Notes Due 2028 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.26161% |  |
Subordinated Debenture | SOFR | 6.75% Subordinated Notes Due 2028 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 4.095% |  |
Interest rate during the period |  |  | 9.73% |  |
Subordinated Debenture | SOFR | Acquired Subordinated Notes | Forecast |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) | 3.11% |  |  |  |
Subordinated Debenture | First Merchant Capital Trust II |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt face amount |  |  | $ 41,700,000 | $ 41,700,000 |
Interest rate during the period |  |  | 7.21% | 6.33% |
Subordinated Debenture | First Merchant Capital Trust II | Three-Month LIBOR |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  |  | 1.56% |
Subordinated Debenture | First Merchant Capital Trust II | SOFR |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 26.161% |  |
Subordinated Debenture | Ameriana Capital Trust I |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt face amount |  |  | $ 10,300,000 | $ 10,300,000 |
Interest rate during the period |  |  | 7.15% | 6.27% |
Subordinated Debenture | Ameriana Capital Trust I | Three-Month LIBOR |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 1.50% | 1.50% |
Subordinated Debenture | Ameriana Capital Trust I | SOFR |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.26161% |  |
Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt face amount |  | $ 70,000,000 |  |  |
Private debt issuance, number of institutional investors | investor |  | 4 |  |  |
Senior Notes | 5.00% Senior Notes Due 2028 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt face amount |  | $ 5,000,000 |  |  |
Interest rate on notes (as a percent) |  | 5.00% |  |  |
Senior Notes | Three-Month SOFR |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.26161% |  |
Senior Notes | Three-Month SOFR | 5.00% Senior Notes Due 2028 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 0.26161% |  |
Senior Notes | SOFR | 5.00% Senior Notes Due 2028 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Basis spread on variable rate (as a percent) |  |  | 2.345% |  |
Interest rate during the period |  |  | 7.98% |  |
Secured Debt |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt outstanding |  |  | $ 7,300,000 |  |
Interest rate on notes (as a percent) |  |  | 3.41% |  |
Secured Debt | Level One |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Debt outstanding |  |  |  | $ 1,200,000 |
Interest rate on notes (as a percent) |  |  | 1.00% |  |

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