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Investment Securities - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities purchased	$ 587,942	$ 530,227	$ 1,064,815
Payments to acquire held-to-maturity securities	195,357	0	0
Proceeds from sales of available-for-sale investment securities	5,020	1,499	$ 204,351
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	$ 5,694	$ 3,459
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	$ 1,108	$ 944
Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration]	Interest receivable	Interest receivable

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Amount, after allowance for credit loss, of accrued interest on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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