Fair Value Measurements - Additional Information (Details) - USD ($)	6 Months Ended
Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Available-for-sale securities purchased	$ 115,931,000	$ 124,651,000
Proceeds from sales of available-for-sale securities	$ 0	4,510,000
Term of contractual due dates of substantially all mortgage-backed securities (in years)	10 years
Accrued interest for available-for-sale securities	$ 6,174,000		$ 5,694,000
Accrued interest for held-to-maturity securities	1,065,000		$ 1,108,000
Proceeds from sale of held-to-maturity securities	0	0
Held-to-maturity securities purchased		$ 0
Consumer loans \| Single-family residential
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Foreclosed residential real estate properties held as REO	121,000
Real estate acquired through foreclosure	$ 2,384,000

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).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure.

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