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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)
 3 Months Ended 6 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Mar. 31, 2024
USD ($)
Mar. 31, 2023
USD ($)
Mar. 31, 2022
USD ($)
Jun. 30, 2024
USD ($)
Jun. 30, 2023
USD ($)
Jun. 30, 2022
USD ($)
Sep. 30, 2024
USD ($)
segment
Sep. 30, 2023
USD ($)
Sep. 30, 2022
USD ($)
Dec. 31, 2023
USD ($)
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                 $ 4,071,071,000 $ 4,363,391,000    
Loans held for sale $ 17,148,000 $ 36,182,000 $ 25,946,000 $ 24,780,000 $ 31,410,000 $ 29,380,000 $ 87,351,000 $ 67,571,000 $ 691,688,000 77,779,000 $ 21,071,000 $ 69,518,000
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                 1,000,000.0 3,300,000    
Impairment on rental equipment $ 24,000 $ 0 $ 2,013,000 $ 2,774,000 $ 0 $ 2,013,000 $ 24,000 $ 0 2,013,000 24,000 $ 0  
Transfers                        
Aggregate unpaid balance of loans serviced for others                 364,500,000 332,500,000    
National Lending | Commercial finance                        
Schedule of Equity Method Investments [Line Items]                        
Total loans and leases                 3,295,599,000 3,726,868,000    
National Lending | Consumer finance                        
Schedule of Equity Method Investments [Line Items]                        
Total loans and leases                 248,800,000 254,416,000    
National Lending | Consumer finance | Consumer Loan Programs                        
Schedule of Equity Method Investments [Line Items]                        
Total loans and leases                 273,000,000.0 $ 332,200,000    
Loans held for sale                 $ 24,200,000      
Percentage of total gross loan portfolio                 6.00% 8.00%    
Loans held for investment                 $ 248,800,000 $ 254,400,000    
Special Purpose Entity                        
Schedule of Equity Method Investments [Line Items]                        
Equity method investments                 5,800,000 1,200,000    
Equity Investments                        
Equity method investments                 5,800,000 1,200,000    
Special Purpose Entity | National Lending | Commercial finance                        
Schedule of Equity Method Investments [Line Items]                        
Total loans and leases                 18,400,000 13,700,000    
Loans held for sale                 4,600,000 0    
FRB                        
Schedule of Equity Method Investments [Line Items]                        
Interest bearing deposits                 104,900,000      
FHLB                        
Schedule of Equity Method Investments [Line Items]                        
Interest bearing deposits                 1,700,000      
Other Assets                        
Schedule of Equity Method Investments [Line Items]                        
Equity method investments                 4,100,000 4,100,000    
Equity Investments                        
Equity method investments                 4,100,000 4,100,000    
Alternative investment                 $ 9,500,000 12,100,000    
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                 $ 11,800,000 8,400,000    
Equity Investments                        
Equity method investments                 $ 11,800,000 $ 8,400,000    
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