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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)
 12 Months Ended
Sep. 30, 2025
USD ($)
segment
Sep. 30, 2024
USD ($)
Sep. 30, 2023
USD ($)
Schedule of Equity Method Investments [Line Items]      
Joint venture ownership interest in each of the joint ventures (as a percentage) 80.00%    
Total loans and leases $ 4,665,006,000 $ 4,071,071,000  
Loans held for sale $ 179,421,000 691,688,000  
Number of reporting segments | segment 3    
Terms of FHLB advances 90 days    
Reserve balances in cash or on deposit with FRB (Federal Reserve Bank) $ 0    
SECURITIES      
Credit loss risk 0    
Equity Investments      
Impairment loss 3,000,000.0 1,000,000.0  
Impairment on rental equipment 2,915,000 2,013,000 $ 24,000
Impairment 0 0 $ 0
Transfers      
Aggregate unpaid balance of loans serviced for others 991,500,000 364,500,000  
National Lending | Commercial finance      
Schedule of Equity Method Investments [Line Items]      
Total loans and leases 3,923,969,000 3,295,599,000  
National Lending | Consumer finance      
Schedule of Equity Method Investments [Line Items]      
Total loans and leases 93,319,000 248,800,000  
National Lending | Consumer finance | Consumer Loan Programs      
Schedule of Equity Method Investments [Line Items]      
Total loans and leases 256,400,000 273,000,000.0  
Loans held for sale $ 163,100,000 $ 24,200,000  
Equity Investments      
Percentage of total gross loan portfolio 5.00% 6.00%  
Loans held for investment $ 93,300,000 $ 248,800,000  
Special Purpose Entity      
Schedule of Equity Method Investments [Line Items]      
Equity method investments 2,300,000 5,800,000  
Equity Investments      
Equity method investments 2,300,000 5,800,000  
Special Purpose Entity | National Lending | Commercial finance      
Schedule of Equity Method Investments [Line Items]      
Total loans and leases 25,800,000 18,400,000  
Loans held for sale 0 4,600,000  
FRB      
Schedule of Equity Method Investments [Line Items]      
Interest bearing deposits 94,600,000    
FHLB      
Schedule of Equity Method Investments [Line Items]      
Interest bearing deposits 1,400,000    
Other Assets      
Schedule of Equity Method Investments [Line Items]      
Equity method investments 5,300,000 4,100,000  
Equity Investments      
Equity method investments 5,300,000 4,100,000  
Alternative investment $ 6,700,000 9,500,000  
Minimum | Other Assets Investments      
Equity Investments      
Equity method investment, ownership percentage 5.00%    
Maximum | Other Assets Investments      
Equity Investments      
Equity method investment, ownership percentage 25.00%    
Fair Value Measured at Net Asset Value Per Share | Other Assets      
Schedule of Equity Method Investments [Line Items]      
Equity method investments $ 13,200,000 11,800,000  
Equity Investments      
Equity method investments $ 13,200,000 $ 11,800,000  
First Midwest Financial Capital Trust I      
Schedule of Equity Method Investments [Line Items]      
Percentage of interest in subsidiary 100.00%    
Buildings      
Property, Plant and Equipment [Line Items]      
Premises, furniture and equipment, estimated useful lives 39 years    
Software Development      
Property, Plant and Equipment [Line Items]      
Premises, furniture and equipment, estimated useful lives 3 years    
Leasehold Improvements | Minimum      
Property, Plant and Equipment [Line Items]      
Premises, furniture and equipment, estimated useful lives 2 years    
Leasehold Improvements | Maximum      
Property, Plant and Equipment [Line Items]      
Premises, furniture and equipment, estimated useful lives 15 years