v3.26.1
BORROWINGS - Narrative (Details) - USD ($)
 6 Months Ended 12 Months Ended
Feb. 01, 2026
Sep. 30, 2024
Jun. 30, 2026
Jun. 30, 2025
Dec. 31, 2025
Debt Instrument [Line Items]          
Total available remaining borrowing capacity from FHLB     $ 749,300,000    
Federal Home Loan Bank advances     1,414,059,000   $ 798,549,000
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   $ 75,000,000.0      
Non-refundable facility fee   0.40%      
Putable Advances          
Debt Instrument [Line Items]          
Federal Home Loan Bank advances     $ 410,000,000.0    
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     $ 120,300,000 $ 169,100,000  
Average balance     110,200,000 $ 141,700,000  
Debt outstanding     103,340,000    
Subordinated Debentures and Other Borrowings          
Debt Instrument [Line Items]          
Debt outstanding     86,350,000   57,600,000
Subordinated Debenture | First Savings Subordinated Notes          
Debt Instrument [Line Items]          
Debt face amount $ 29,000,000.0        
Basis spread on variable rate (as a percent) 2.76%        
Interest rate on notes (as a percent) 4.50%        
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     5.49%   5.54%
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     5.43%   5.48%
Secured Debt          
Debt Instrument [Line Items]          
Debt outstanding     $ 6,900,000   $ 7,000,000.0
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%