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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)
 12 Months Ended
Sep. 30, 2023
USD ($)
segment
Sep. 30, 2022
USD ($)
Sep. 30, 2021
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 $ 4,359,681,000 $ 3,529,280,000  
Loans held for sale $ 77,779,000 21,071,000  
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    
Impairment 0 0 $ 0
SECURITIES      
Credit loss risk 0    
Equity Investments      
Impairment loss 3,300,000    
Transfers      
Aggregate unpaid balance of loans serviced for others 332,500,000 336,600,000  
National Lending | Commercial finance      
Schedule of Equity Method Investments [Line Items]      
Total loans and leases 3,723,158,000 3,023,673,000  
Special Purpose Entity      
Schedule of Equity Method Investments [Line Items]      
Equity method investments 1,200,000    
Equity Investments      
Equity method investments 1,200,000    
Special Purpose Entity | National Lending | Commercial finance      
Schedule of Equity Method Investments [Line Items]      
Total loans and leases 13,700,000    
Loans held for sale 0    
FRB      
Schedule of Equity Method Investments [Line Items]      
Interest bearing deposits 260,300,000    
FHLB      
Schedule of Equity Method Investments [Line Items]      
Interest bearing deposits $ 1,900,000    
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%    
Other Assets      
Schedule of Equity Method Investments [Line Items]      
Equity method investments $ 4,100,000 2,900,000  
Equity Investments      
Equity method investments 4,100,000 2,900,000  
Alternative investment 12,100,000 15,300,000  
Alternative investment, fair value decrease   1,000,000  
Impairment loss   0  
Fair Value Measured at Net Asset Value Per Share | Other Assets      
Schedule of Equity Method Investments [Line Items]      
Equity method investments 8,400,000 7,200,000  
Equity Investments      
Equity method investments $ 8,400,000 $ 7,200,000  
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