Loans Receivable and Related Allowance for Loan Losses - Nonaccrual Loans by Aging Category (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	$ 3,003	$ 3,278
Not Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	1,284	1,100
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	77
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	75	557
90 Days and Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	1,644	1,544
Residential first mortgages
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	856	1,126
Residential first mortgages | Not Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	366	72
Residential first mortgages | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	77
Residential first mortgages | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	75	0
Residential first mortgages | 90 Days and Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	415	977
Home equity loans and lines of credit
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	526	340
Home equity loans and lines of credit | Not Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	8	0
Home equity loans and lines of credit | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	0
Home equity loans and lines of credit | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	0
Home equity loans and lines of credit | 90 Days and Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	518	340
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	964	1,073
Commercial real estate | Not Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	341	397
Commercial real estate | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	0
Commercial real estate | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	557
Commercial real estate | 90 Days and Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	623	119
Commercial business
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	584	726
Commercial business | Not Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	569	631
Commercial business | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	0
Commercial business | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	0
Commercial business | 90 Days and Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	15	95
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	73	13
Consumer | Not Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	0
Consumer | 30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	0
Consumer | 60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	0	0
Consumer | 90 Days and Greater Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans	$ 73	$ 13