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	Sep. 30, 2024	Dec. 31, 2023
Unadvanced portions of loans [Member]
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items]
Unadvanced portions of loans	$ 41,135	$ 46,175
Commitments to originate loans [Member]
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items]
Unadvanced portions of loans	25,095	34,074
Standby Letters of Credit [Member]
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items]
Unadvanced portions of loans	$ 125	$ 125