BORROWINGS - Narrative (Details)			9 Months Ended		12 Months Ended
	Dec. 18, 2024	Nov. 01, 2013 USD ($) investor	Sep. 30, 2025 USD ($)	Sep. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)	Mar. 31, 2025 USD ($)	Dec. 18, 2019 USD ($)
Debt Instrument [Line Items]
Total available remaining borrowing capacity from FHLB			$ 781,200,000
Putable advances with the FHLB			798,626,000		$ 822,554,000
Debt face amount		$ 70,000,000
Private debt issuance, number of institutional investors | investor		4
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		0
Maximum borrowing capacity				$ 75,000,000
Non-refundable facility fee				0.40%
Putable Advances
Debt Instrument [Line Items]
Putable advances with the FHLB			$ 280,000,000
Minimum
Debt Instrument [Line Items]
Interest rate, minimum (as a percent)			0.00%
Maximum
Debt Instrument [Line Items]
Interest rate, minimum (as a percent)			4.56%
Securities Sold Under Repurchase Agreements
Debt Instrument [Line Items]
Maximum amount of outstanding agreements			$ 169,100,000	$ 194,200,000
Average balance			137,800,000	138,100,000
Debt outstanding			122,226,000
Subordinated Debentures and Term Loans
Debt Instrument [Line Items]
Debt outstanding			57,632,000		93,500,000
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%
Fixed interest rate term		10 years
Redeemed				$ 65,000,000
Subordinated Debenture | Level One Subordinated Notes
Debt Instrument [Line Items]
Debt face amount							$ 30,000,000
Basis spread on variable rate (as a percent)	3.11%
Interest rate on notes (as a percent)							4.75%
Redeemed						$ 30,000,000
Principal amount outstanding			0
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%		0.26161%
Interest rate during the period			5.86%		6.18%
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%		0.26161%
Interest rate during the period			5.80%		6.12%
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 | First Merchants Senior Notes and Subordinated Notes
Debt Instrument [Line Items]
Debt face amount			$ 0
Redeemed			5,000,000
Weighted average interest rate					7.18%
Senior Notes | First Merchants Senior Notes and Subordinated Notes | Secured Overnight Financing Rate (SOFR)
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)					0.26161%
Senior Notes | First Merchants Senior Notes and Subordinated Notes | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)					2.345%
Secured Debt
Debt Instrument [Line Items]
Debt outstanding			$ 7,000,000.0		$ 7,100,000
Interest rate on notes (as a percent)			3.41%		3.41%
Secured Debt | Level One
Debt Instrument [Line Items]
Debt outstanding			$ 1,100,000		$ 1,100,000
Interest rate on notes (as a percent)			1.00%		1.00%