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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)
 12 Months Ended
Sep. 30, 2021
USD ($)
joint_venture
segment
Sep. 30, 2020
USD ($)
Sep. 30, 2019
USD ($)
Oct. 01, 2020
USD ($)
Sep. 30, 2018
USD ($)
Investment Holdings [Line Items]          
Number of joint venture LLC's | joint_venture 5        
Joint venture ownership interest in each of the joint ventures (as a percentage) 80.00%        
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 $ 0      
Impairment 0 0 $ 0    
Total stockholders' equity 871,884,000 847,308,000 843,958,000   $ 747,726,000
Loans outstanding with individuals 0 0      
Allowance for credit losses 68,281,000 56,188,000 29,149,000   13,040,000
SECURITIES          
Other-than-temporary impairment recorded 0        
Equity Investments          
Impairment loss 2,600,000        
Net income before noncontrolling interest 145,745,000 109,344,000 101,316,000    
Transfers          
Aggregate unpaid balance of loans serviced for others 307,300,000 232,300,000      
Unfunded Loan Commitment          
Investment Holdings [Line Items]          
Allowance for credit losses 690,000 32,000      
Equity Method Investment, Nonconsolidated Investee          
Equity Investments          
Net income before noncontrolling interest 300,000        
Adjustment          
Investment Holdings [Line Items]          
Allowance for credit losses   12,773,000 0 $ 12,800,000 0
Adjustment | Unfunded Loan Commitment          
Investment Holdings [Line Items]          
Allowance for credit losses   831,000      
FRB          
Investment Holdings [Line Items]          
Interest bearing deposits 184,700,000        
FHLB          
Investment Holdings [Line Items]          
Interest bearing deposits $ 2,300,000        
Minimum          
Equity Investments          
Equity method investment, ownership percentage 5.00%        
Maximum          
Equity Investments          
Equity method investment, ownership percentage 25.00%        
Other Assets          
Equity Investments          
Equity method investments $ 3,100,000 11,000,000      
Other Assets | Accounting Standards Update 2016-01          
Equity Investments          
Impairment loss   1,300,000      
Alternative investment 12,900,000 12,000,000      
Alternative investment, fair value increase (decrease) 8,000,000 0      
Fair Value Measured at Net Asset Value Per Share | Other Assets          
Equity Investments          
Equity method investments 4,600,000 2,800,000      
Retained Earnings          
Investment Holdings [Line Items]          
Total stockholders' equity 259,189,000 234,927,000 252,813,000   213,048,000
Equity Investments          
Net income before noncontrolling interest 141,708,000 104,720,000 97,004,000    
Retained Earnings | Adjustment          
Investment Holdings [Line Items]          
Total stockholders' equity   (8,400,000)      
Retained Earnings | Accounting Standards Update 2016-01 | Adjustment          
Investment Holdings [Line Items]          
Total stockholders' equity         475,000
Non-controlling interest          
Investment Holdings [Line Items]          
Total stockholders' equity 1,155,000 3,603,000 4,047,000   $ 3,574,000
Equity Investments          
Net income before noncontrolling interest $ 4,037,000 4,624,000 $ 4,312,000    
Non-controlling interest | Adjustment          
Investment Holdings [Line Items]          
Total stockholders' equity   $ (2,500,000)      
First Midwest Financial Capital Trust I          
Investment Holdings [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        
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