Commitments and Contingencies - Summary of Financial Instruments Outstanding Contract Amounts Represent Credit Risk (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Commitments to Originate Loans [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Financial instruments with off-balance-sheet risk	$ 251,697	$ 411,401
Unused Lines and Letters of Credit [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Financial instruments with off-balance-sheet risk	75,681	71,458
Unadvanced Funds on Construction Loans [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Financial instruments with off-balance-sheet risk	12,343	9,110
Overdraft Lines of Credit [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Financial instruments with off-balance-sheet risk	$ 7,910	$ 7,969