Investment Securities - Narrative (Details) - USD ($)	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities purchased	$ 376,481,000	$ 587,942,000	$ 530,227,000
Payments to acquire held-to-maturity securities	0	195,357,000	0
Proceeds from sales of available-for-sale investment securities	1,169,000	5,020,000	$ 1,499,000
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	$ 8,641,000	$ 5,694,000
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	$ 1,013,000	$ 1,108,000
Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Interest receivable	Interest receivable