BORROWINGS - Narrative (Details)			9 Months Ended		12 Months Ended
	Dec. 18, 2024	Nov. 01, 2013 USD ($) investor	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Dec. 18, 2019 USD ($)
Debt Instrument [Line Items]
Total available remaining borrowing capacity from FHLB			$ 762,600,000
Federal Home Loan Bank advances			$ 832,629,000		$ 712,852,000
Debt face amount		$ 70,000,000
Private debt issuance, number of institutional investors | investor		4
5.00% Senior Notes Due 2028
Debt Instrument [Line Items]
Redemption price (percent)		100.00%
Senior Notes
Debt Instrument [Line Items]
Fixed interest rate term		10 years
U.S. Bank, N.A | Line of Credit
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)			2.25%
Outstanding balance on the line of credit			$ 0
Line of credit facility, maximum borrowing capacity			$ 75,000,000
Line of credit facility, commitment fee percentage			0.40%
Putable Advances
Debt Instrument [Line Items]
Federal Home Loan Bank advances			$ 305,000,000
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	$ 241,900,000
Total of average agreements			138,100,000	$ 181,200,000
Debt outstanding			124,894,000
Subordinated Debentures and Term Loans
Debt Instrument [Line Items]
Debt outstanding			93,562,000		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 of intent to redeem
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 | 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
Basis spread on variable rate (as a percent)			0.26161%
Interest rate during the period			6.77%		7.21%
Subordinated Debenture | Ameriana Capital Trust I
Debt Instrument [Line Items]
Debt face amount			$ 10,300,000		$ 10,300,000
Basis spread on variable rate (as a percent)			0.26161%
Interest rate during the period			6.71%		7.15%
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%
Secured Debt
Debt Instrument [Line Items]
Debt outstanding			$ 7,200,000		$ 7,300,000
Interest rate on notes (as a percent)			3.41%		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%		1.00%