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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, 2020 | Sep. 30, 2019 | Jun. 30, 2020
Financing Receivable, Impaired [Line Items] |  |  |
Specific Allowance | $ 0 | $ 19 |
Recorded Balance | 1,336 | 1,813 | $ 1,660
Unpaid Principal Balance | 1,336 | 1,813 | 1,660
Specific Allowance | 0 | 19 |
Average Investment in Impaired Loans | 1,345 | 1,831 | 1,800
Interest Income Recognized | 19 | 15 | 84
Interest on Cash Basis | 17 | 15 | 87
Commercial [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Balance, Loans without a specific allowance |  | 16 |
Unpaid Principal Balance, Loans without a specific allowance |  | 16 |
Average Investment in Impaired Loans, Loans without a specific allowance |  | 17 | 3
Recorded Balance |  | 16 |
Unpaid Principal Balance |  | 16 |
Average Investment in Impaired Loans |  | 17 | 3
Commercial [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Balance, Loans without a specific allowance | 55 | 55 | 304
Unpaid Principal Balance, Loans without a specific allowance | 55 | 55 | 304
Average Investment in Impaired Loans, Loans without a specific allowance | 57 | 58 | 382
Interest Income Recognized, Loans without a specific allowance |  |  | 23
Interest on Cash Basis, Loans without a specific allowance |  |  | 25
Recorded Balance | 55 | 55 | 304
Unpaid Principal Balance | 55 | 55 | 304
Average Investment in Impaired Loans | 57 | 58 | 382
Interest Income Recognized |  |  | 23
Interest on Cash Basis |  |  | 25
Consumer [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Balance, Loans without a specific allowance | 4 | 15 | 5
Unpaid Principal Balance, Loans without a specific allowance | 4 | 15 | 5
Average Investment in Impaired Loans, Loans without a specific allowance | 5 | 17 | 9
Recorded Balance, Loans with a specific allowance | 0 | 6 |
Unpaid Principal Balance, Loans with a specific allowance | 0 | 6 |
Specific Allowance | 0 | 6 |
Average Investment in Impaired Loans, Loans with a specific allowance | 0 | 7 |
Interest Income Recognized, Loans with a specific allowance | 0 |  |
Interest on Cash Basis, Loans with a specific allowance | 0 |  |
Recorded Balance | 4 | 21 | 5
Unpaid Principal Balance | 4 | 21 | 5
Specific Allowance | 0 | 6 |
Average Investment in Impaired Loans | 5 | 24 | 9
One- to four-family [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Balance, Loans without a specific allowance | 1,277 | 1,656 | 1,336
Unpaid Principal Balance, Loans without a specific allowance | 1,277 | 1,656 | 1,336
Average Investment in Impaired Loans, Loans without a specific allowance | 1,283 | 1,666 | 1,388
Interest Income Recognized, Loans without a specific allowance | 19 | 15 | 61
Interest on Cash Basis, Loans without a specific allowance | 17 | 15 | 62
Recorded Balance, Loans with a specific allowance | 0 | 46 |
Unpaid Principal Balance, Loans with a specific allowance | 0 | 46 |
Specific Allowance | 0 | 13 |
Average Investment in Impaired Loans, Loans with a specific allowance | 0 | 46 |
Interest Income Recognized, Loans with a specific allowance | 0 |  |
Interest on Cash Basis, Loans with a specific allowance | 0 |  |
Recorded Balance | 1,277 | 1,702 | 1,336
Unpaid Principal Balance | 1,277 | 1,702 | 1,336
Specific Allowance | 0 | 13 |
Average Investment in Impaired Loans | 1,283 | 1,712 | 1,388
Interest Income Recognized | 19 | 15 | 61
Interest on Cash Basis | $ 17 | 15 | 62
Home equity lines of credit [Member] |  |  |
Financing Receivable, Impaired [Line Items] |  |  |
Recorded Balance, Loans without a specific allowance |  | 19 | 15
Unpaid Principal Balance, Loans without a specific allowance |  | 19 | 15
Average Investment in Impaired Loans, Loans without a specific allowance |  | 20 | 18
Recorded Balance |  | 19 | 15
Unpaid Principal Balance |  | 19 | 15
Average Investment in Impaired Loans |  | $ 20 | $ 18

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