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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details)
 12 Months Ended
Sep. 30, 2020
USD ($)
segment
joint_venture
Sep. 30, 2019
USD ($)
segment
Sep. 30, 2018
USD ($)
Oct. 01, 2019
USD ($)
Oct. 01, 2018
USD ($)
Sep. 30, 2017
USD ($)
NATURE OF BUSINESS AND INDUSTRY SEGMENT INFORMATION [Abstract]            
Number of reporting segments | segment 3 3        
CASH AND CASH EQUIVALENTS AND FEDERAL FUNDS SOLD [Abstract]            
Terms of FHLB advances 90 days          
Reserve balances in cash or on deposit with FRB (Federal Reserve Bank) $ 0 $ 33,900,000        
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%          
Federal funds sold $ 0          
Impairment 0 0 $ 0      
Total stockholders' equity 847,308,000 843,958,000 747,726,000     $ 434,496,000
Loans outstanding with individuals 0 5,100,000        
Operating lease, right-of-use asset 25,800,000          
Operating lease liability 27,073,000          
SECURITIES [Abstract]            
Other-than-temporary impairment recorded 0 0 0      
Equity Investments            
Impairment loss 1,300,000          
Net income before noncontrolling interest 109,344,000 101,316,000 52,293,000      
Transfers            
Aggregate unpaid balance of loans serviced for others 232,300,000 175,500,000        
Equity Method Investment, Nonconsolidated Investee            
Equity Investments            
Net income before noncontrolling interest 2,600,000          
FRB            
Investment Holdings [Line Items]            
Interest bearing deposits 362,000,000.0          
FHLB            
Investment Holdings [Line Items]            
Interest bearing deposits $ 2,000,000.0          
Minimum            
Equity Investments            
Equity method investment, ownership percentage 5.00%          
ALLOWANCE FOR LOAN LOSSES [Abstract]            
Look back period 3 years          
Maximum            
Equity Investments            
Equity method investment, ownership percentage 25.00%          
ALLOWANCE FOR LOAN LOSSES [Abstract]            
Look back period 7 years          
Accounting Standards Update 2016-02            
Investment Holdings [Line Items]            
Operating lease, right-of-use asset       $ 27,400,000    
Operating lease liability       $ 28,600,000    
Accounting Standards Update 2014-09 | Adjustment            
Investment Holdings [Line Items]            
Total stockholders' equity     1,502,000      
Other Assets            
Equity Investments            
Equity method investments $ 11,000,000.0          
Other Assets | Accounting Standards Update 2016-01            
Equity Investments            
Alternative Investment 12,000,000.0          
Fair Value Measured at Net Asset Value Per Share | Other Assets            
Equity Investments            
Equity method investments 2,800,000          
      Retained Earnings            
Investment Holdings [Line Items]            
Total stockholders' equity 234,927,000 252,813,000 213,048,000     $ 167,164,000
Equity Investments            
Net income before noncontrolling interest $ 104,720,000 $ 97,004,000 51,620,000      
      Retained Earnings | Accounting Standards Update 2016-01 | Adjustment            
Investment Holdings [Line Items]            
Total stockholders' equity     475,000      
      Retained Earnings | Accounting Standards Update 2014-09 | Adjustment            
Investment Holdings [Line Items]            
Total stockholders' equity     $ 1,502,000   $ 1,500,000  
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