Loans - Summary of Information on Non-accrual Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Non Performing Loans [Line Items]
Total non-accrual loans	$ 1,417	$ 2,013
Total non-accrual loans to total loans	0.43%	0.64%
Total non-accrual loans to total assets	0.28%	0.47%
Residential Real Estate First Mortgages [Member]
Non Performing Loans [Line Items]
Total non-accrual loans	$ 1,274	$ 1,690
Consumer Home Equity and Lines of Credit [Member]
Non Performing Loans [Line Items]
Total non-accrual loans	$ 143	143
Commercial Real Estate [Member]
Non Performing Loans [Line Items]
Total non-accrual loans		$ 180