Loans - Summary of Information on Non-accrual Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Non Performing Loans [Line Items]
Total non-accrual loans	$ 1,632	$ 1,887
Total non-accrual loans to total loans	0.44%	0.57%
Total non-accrual loans to total assets	0.34%	0.40%
Commercial Land [Member]
Non Performing Loans [Line Items]
Total non-accrual loans	$ 303	$ 303
Commercial Loan Other [Member]
Non Performing Loans [Line Items]
Total non-accrual loans	20	32
Residential Real Estate First Mortgages [Member]
Non Performing Loans [Line Items]
Total non-accrual loans	1,120	1,128
Consumer Home Equity and Lines of Credit [Member]
Non Performing Loans [Line Items]
Total non-accrual loans	$ 189	420
Consumer Other [Member]
Non Performing Loans [Line Items]
Total non-accrual loans		$ 4