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%