Financial Instruments with Off-Balance-Sheet Risk - Summary of Financial Instruments with Off-Balance-Sheet Risk (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	$ 6,796	$ 4,936
Commitments to Originate 1-4 Family Mortgages [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	13,877	5,638
Unused Lines of Credit [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	362,357	320,874
Unadvanced Portions of Construction Loans [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	22,049	11,589
Unadvanced Portions of Other Loans [Member]
Financial instruments whose contract amount represents credit risk:
Financial instruments with off-balance-sheet risk	$ 52,224	$ 41,717