Commitments and Contingencies - Summary of Financial Instrument Liabilities Represent Off-Balance Sheet Credit Risk (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Loss Contingencies [Line Items]
Off-balance-sheet, credit risk	$ 159,355	$ 154,507
Commitments To Originate Loans
Loss Contingencies [Line Items]
Off-balance-sheet, credit risk	33,370	22,701
Commitment To Purchase Loans
Loss Contingencies [Line Items]
Off-balance-sheet, credit risk	920	415
Unadvanced Funds On Lines Of Credit
Loss Contingencies [Line Items]
Off-balance-sheet, credit risk	78,041	78,378
Unadvanced Funds on Construction Loans
Loss Contingencies [Line Items]
Off-balance-sheet, credit risk	46,974	53,013
Letters of Credit
Loss Contingencies [Line Items]
Off-balance-sheet, credit risk	$ 50	$ 0