v3.26.3
BORROWINGS - Narrative (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 25, 2026
Feb. 01, 2026
Sep. 30, 2024
Mar. 31, 2026
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Debt Instrument [Line Items]              
Total available remaining borrowing capacity from FHLB         $ 1,400,000,000    
Federal Home Loan Bank advances         903,941,000   $ 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         $ 130,000,000.0    
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         108,900,000 $ 137,800,000  
Debt outstanding         99,638,000    
Subordinated Notes, Debentures and Other Borrowings              
Debt Instrument [Line Items]              
Debt outstanding         184,505,000   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 | New Suboridinated Debt              
Debt Instrument [Line Items]              
Debt face amount $ 100,000,000.0            
Basis spread on variable rate (as a percent) 2.02%            
Interest rate on notes (as a percent) 6.75%            
Debt issuance costs $ 1,900,000            
Proceeds from issuance of subordinated debt $ 98,100,000            
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         5.72%   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         5.66%   5.48%
Secured Debt              
Debt Instrument [Line Items]              
Debt outstanding         $ 6,800,000   $ 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%