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Loans and Allowance for Loan Losses - Summary of Impaired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021
Financing Receivable, Impaired [Line Items]
Specific Allowance	$ 0	$ 0	$ 0
Recorded Balance	1,129	1,336	1,298
Unpaid Principal Balance	1,129	1,336	1,298
Specific Allowance	0	0	0
Average Investment in Impaired Loans	1,132	1,345	1,536
Interest Income Recognized	6	19	86
Interest on Cash Basis	5	17	73
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Recorded Balance, Loans without a specific allowance		0	0
Unpaid Principal Balance, Loans without a specific allowance		0	0
Average Investment in Impaired Loans, Loans without a specific allowance		0	0
Specific Allowance		0	0
Recorded Balance		0	0
Unpaid Principal Balance		0	0
Specific Allowance		0	0
Average Investment in Impaired Loans		0	0
Interest Income Recognized		0	0
Interest on Cash Basis		0	0
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Recorded Balance, Loans without a specific allowance	43	55	46
Unpaid Principal Balance, Loans without a specific allowance	43	55	46
Average Investment in Impaired Loans, Loans without a specific allowance	44	57	265
Interest Income Recognized, Loans without a specific allowance	1	0	19
Interest on Cash Basis, Loans without a specific allowance	1	0	23
Recorded Balance	43	55	0
Unpaid Principal Balance	43	55	0
Average Investment in Impaired Loans	44	57	0
Interest Income Recognized	1	0	0
Interest on Cash Basis	1	0	0
Consumer [Member]
Financing Receivable, Impaired [Line Items]
Recorded Balance, Loans without a specific allowance		4	0
Unpaid Principal Balance, Loans without a specific allowance		4	0
Average Investment in Impaired Loans, Loans without a specific allowance		5	0
Interest Income Recognized, Loans without a specific allowance	0	0	0
Interest on Cash Basis, Loans without a specific allowance	0	0	0
Recorded Balance, Loans with a specific allowance		0	0
Unpaid Principal Balance, Loans with a specific allowance		0	0
Specific Allowance		0	0
Average Investment in Impaired Loans, Loans with a specific allowance		0	0
Interest Income Recognized, Loans with a specific allowance		0	0
Interest on Cash Basis, Loans with a specific allowance		0	0
Recorded Balance		4	46
Unpaid Principal Balance		4	46
Specific Allowance		0	0
Average Investment in Impaired Loans		5	265
Interest Income Recognized		0	19
Interest on Cash Basis		0	23
One- to four-family [Member]
Financing Receivable, Impaired [Line Items]
Recorded Balance, Loans without a specific allowance	1,086	1,277	1,252
Unpaid Principal Balance, Loans without a specific allowance	1,086	1,277	1,252
Average Investment in Impaired Loans, Loans without a specific allowance	1,088	1,283	1,271
Interest Income Recognized, Loans without a specific allowance	5	19	67
Interest on Cash Basis, Loans without a specific allowance	4	17	50
Recorded Balance, Loans with a specific allowance		0	0
Unpaid Principal Balance, Loans with a specific allowance		0	0
Specific Allowance		0	0
Average Investment in Impaired Loans, Loans with a specific allowance		0	0
Interest Income Recognized, Loans with a specific allowance		0	0
Interest on Cash Basis, Loans with a specific allowance		0	0
Recorded Balance	1,086	1,277
Unpaid Principal Balance	1,086	1,277
Specific Allowance		0	0
Average Investment in Impaired Loans	1,088	1,283
Interest Income Recognized	5	19
Interest on Cash Basis	$ 4	17
Multi-family [Member]
Financing Receivable, Impaired [Line Items]
Recorded Balance		0	1,252
Unpaid Principal Balance		0	1,252
Average Investment in Impaired Loans		0	1,271
Interest Income Recognized		0	67
Interest on Cash Basis		0	50
Home equity lines of credit [Member]
Financing Receivable, Impaired [Line Items]
Recorded Balance, Loans without a specific allowance		0	0
Unpaid Principal Balance, Loans without a specific allowance		0	0
Average Investment in Impaired Loans, Loans without a specific allowance		0	0
Interest Income Recognized, Loans without a specific allowance		0	0
Interest on Cash Basis, Loans without a specific allowance		0	0
Recorded Balance		0	0
Unpaid Principal Balance		0	0
Average Investment in Impaired Loans		0	0
Interest Income Recognized		0	0
Interest on Cash Basis		$ 0	$ 0

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