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SECURITIES - Narrative (Details)
 6 Months Ended
Mar. 31, 2024
USD ($)
security
Mar. 31, 2023
USD ($)
Sep. 30, 2023
USD ($)
Debt Securities, Available-for-sale [Line Items]      
AFS number of securities in an unrealized loss position (in securities) | security 194    
Federal reserve bank stock $ 19,700,000   $ 19,700,000
Federal home loan bank stock 6,200,000   8,500,000
Impairment recognized 0 $ 500,000  
Marketable      
Debt Securities, Available-for-sale [Line Items]      
Equity securities 4,300,000   3,400,000
Unrealized losses on equity securities 0 0  
Non-marketable      
Debt Securities, Available-for-sale [Line Items]      
Equity securities 10,000,000   8,400,000
Unrealized losses on equity securities 400,000    
Unrealized gains on equity securities 2,500,000 $ 100,000  
Equity securities without readily determinable fair value, amount $ 14,800,000   $ 16,200,000
Number of equity securities sold | security 1