Loans - Summary of Information Regarding Non-accrual Loans (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) Contract	Dec. 31, 2017 USD ($) Contract
Financing Receivable, Modifications [Line Items]
Amount | $	$ 1,138	$ 1,558
Number | Contract	8	10
Accruing Loans [Member]
Financing Receivable, Modifications [Line Items]
Amount | $	$ 462	$ 729
Number | Contract	2	4
Non Accruing Loan [Member]
Financing Receivable, Modifications [Line Items]
Amount | $	$ 676	$ 829
Number | Contract	6	6
Residential Real Estate First Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Amount | $	$ 1,106	$ 1,524
Number | Contract	7	9
Residential Real Estate First Mortgages [Member] | Accruing Loans [Member]
Financing Receivable, Modifications [Line Items]
Amount | $	$ 462	$ 729
Number | Contract	2	4
Residential Real Estate First Mortgages [Member] | Non Accruing Loan [Member]
Financing Receivable, Modifications [Line Items]
Amount | $	$ 644	$ 795
Number | Contract	5	5
Consumer Home Equity and Lines of Credit [Member]
Financing Receivable, Modifications [Line Items]
Amount | $	$ 32	$ 34
Number | Contract	1	1
Consumer Home Equity and Lines of Credit [Member] | Non Accruing Loan [Member]
Financing Receivable, Modifications [Line Items]
Amount | $	$ 32	$ 34
Number | Contract	1	1