Financial Instruments with Off-Balance-Sheet Risk - Summary of Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	$ 5,779	$ 4,258
Commitments to Originate 1-4 Family Mortgages [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	13,806	5,075
Unused Lines of Credit [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	625,524	553,045
Unadvanced Portions of Construction Loans [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	11,062	28,746
Unadvanced Portions of Other Loans [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	$ 15,801	$ 20,305