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Financial Instruments with Off-Balance Sheet Credit Exposures - Schedule of Notional Amounts of Financial Instruments with Off-Balance Sheet Credit Risk (Detail) - USD ($) $ in Thousands | Jun. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Unadvanced portions of loans [Member] |  |
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items] |  |
Unadvanced portions of loans | $ 41,736 | $ 42,883
Commitments to originate loans [Member] |  |
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items] |  |
Unadvanced portions of loans | 6,495 | 9,333
Standby Letters of Credit [Member] |  |
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items] |  |
Unadvanced portions of loans | $ 1,738 | $ 125

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