BORROWINGS - Narrative (Details)			3 Months Ended
	Dec. 18, 2024	Nov. 01, 2013 USD ($) investor	Mar. 31, 2024 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	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			$ 721,200,000
Putable advances with the FHLB			612,778,000		$ 712,852,000
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			$ 110,000,000
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			$ 194,200,000	$ 242,200,000
Total of average agreements			172,700,000	$ 208,000,000
Debt outstanding			130,264,000
Subordinated Debentures and Term Loans
Debt Instrument [Line Items]
Debt outstanding			118,612,000		158,600,000
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%
Issued notice			40,000,000
Subordinated Debenture | 6.75% Subordinated Notes Due 2028 | Notice
Debt Instrument [Line Items]
Issued notice			$ 25,000,000
Subordinated Debenture | Level One 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			9.66%
Subordinated Debenture | SOFR | Level One 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			7.15%	7.21%
Subordinated Debenture | First Merchant Capital Trust II | SOFR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)			0.26161%	0.26161%
Subordinated Debenture | Ameriana Capital Trust I
Debt Instrument [Line Items]
Debt face amount			$ 10,300,000		10,300,000
Interest rate during the period			7.09%	7.15%
Subordinated Debenture | Ameriana Capital Trust I | SOFR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)			0.26161%	0.26161%
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 | 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			7.91%
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		$ 1,200,000
Interest rate on notes (as a percent)			1.00%