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Investment Securities - Narrative (Details) - USD ($)
 12 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract]      
Available-for-sale securities purchased $ 549,159,000 $ 376,481,000 $ 587,942,000
Payments to acquire held-to-maturity securities 47,092,000 0 195,357,000
Proceeds from sales of available-for-sale investment securities 182,682,000 1,169,000 $ 5,020,000
Proceeds from sale of held-to-maturity securities $ 0    
Term of contractual due dates (in years) 25 years    
Accrued interest for available-for-sale securities $ 9,311,000 $ 8,641,000  
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] Interest receivable Interest receivable  
Accrued interest for held-to-maturity securities $ 1,154,000 $ 1,013,000  
Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] Interest receivable Interest receivable