Loans Receivable and Related Allowance for Loan Losses - Impaired Loans by Class (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Impaired Loans with Specific Allowance
Unpaid Principal Balance	$ 83	$ 1,313
Recorded Investment	83	1,280
Related Allowance	7	120
Average Recorded Investment	319	677
Interest Income Recognized in Period	3	29
Cash Basis Interest Recognized in Period	3	6
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	2,119	727
Recorded Investment	1,833	553
Average Recorded Investment	1,642	1,080
Interest Income Recognized in Period	14	5
Cash Basis Interest Recognized in Period	14	5
Residential first mortgages
Impaired Loans with Specific Allowance
Unpaid Principal Balance	75	168
Recorded Investment	75	135
Related Allowance	7	19
Average Recorded Investment	88	119
Interest Income Recognized in Period	3	6
Cash Basis Interest Recognized in Period	3	6
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	461	0
Recorded Investment	350	0
Average Recorded Investment	289	23
Interest Income Recognized in Period	8	0
Cash Basis Interest Recognized in Period	8	0
Home equity loans and lines of credit
Impaired Loans with Specific Allowance
Unpaid Principal Balance	8	0
Recorded Investment	8	0
Related Allowance	0	0
Average Recorded Investment	2	0
Interest Income Recognized in Period	0	0
Cash Basis Interest Recognized in Period	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	0	0
Recorded Investment	0	0
Average Recorded Investment	0	0
Interest Income Recognized in Period	0	0
Cash Basis Interest Recognized in Period	0	0
Commercial real estate
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	557
Recorded Investment	0	557
Related Allowance	0	95
Average Recorded Investment	111	130
Interest Income Recognized in Period	0	23
Cash Basis Interest Recognized in Period	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	1,089	631
Recorded Investment	914	457
Average Recorded Investment	855	735
Interest Income Recognized in Period	3	3
Cash Basis Interest Recognized in Period	3	3
Commercial business
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	588
Recorded Investment	0	588
Related Allowance	0	6
Average Recorded Investment	118	428
Interest Income Recognized in Period	0	0
Cash Basis Interest Recognized in Period	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	569	96
Recorded Investment	569	96
Average Recorded Investment	498	322
Interest Income Recognized in Period	3	2
Cash Basis Interest Recognized in Period	3	2
Consumer
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	0
Recorded Investment	0	0
Related Allowance	0	0
Average Recorded Investment	0	0
Interest Income Recognized in Period	0	0
Cash Basis Interest Recognized in Period	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	0	0
Recorded Investment	0	0
Average Recorded Investment	0	0
Interest Income Recognized in Period	0	0
Cash Basis Interest Recognized in Period	$ 0	$ 0

Impaired Financing Receivable, with No Related Allowance [Abstract]

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Impaired Financing Receivable, with Related Allowance [Abstract]

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Amount of allowance for credit losses related to recorded investment.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
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Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5258-111524

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