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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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