Loans Receivable and Related Allowance for Loan Losses - Impaired Loans by Class (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Impaired Loans with Specific Allowance
Unpaid Principal Balance	$ 83	$ 77	$ 83
Recorded Investment	83	77	83
Related Allowance	6	10	7
Average Recorded Investment	83	679	319
Interest Income Recognized in Period	1	1	3
Cash Basis Interest Recognized in Period	1	1	3
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	1,347	2,215	2,119
Recorded Investment	1,235	2,004	1,833
Average Recorded Investment	1,534	1,279	1,642
Interest Income Recognized in Period	2	8	14
Cash Basis Interest Recognized in Period	2	8	14
Residential first mortgages
Impaired Loans with Specific Allowance
Unpaid Principal Balance	75	77	75
Recorded Investment	75	77	75
Related Allowance	6	10	7
Average Recorded Investment	75	106	88
Interest Income Recognized in Period	1	1	3
Cash Basis Interest Recognized in Period	1	1	3
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	456	409	461
Recorded Investment	344	372	350
Average Recorded Investment	347	186	289
Interest Income Recognized in Period	1	7	8
Cash Basis Interest Recognized in Period	1	7	8
Home equity loans and lines of credit
Impaired Loans with Specific Allowance
Unpaid Principal Balance	8	0	8
Recorded Investment	8	0	8
Related Allowance	0	0	0
Average Recorded Investment	8	0	2
Interest Income Recognized in Period	0	0	0
Cash Basis Interest Recognized in Period	0	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized in Period	0	0	0
Cash Basis Interest Recognized in Period	0	0	0
Commercial real estate
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Related Allowance	0	0	0
Average Recorded Investment	0	279	111
Interest Income Recognized in Period	0	0	0
Cash Basis Interest Recognized in Period	0	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	336	1,165	1,089
Recorded Investment	336	991	914
Average Recorded Investment	625	724	855
Interest Income Recognized in Period	0	1	3
Cash Basis Interest Recognized in Period	0	1	3
Commercial business
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Related Allowance	0	0	0
Average Recorded Investment	0	294	118
Interest Income Recognized in Period	0	0	0
Cash Basis Interest Recognized in Period	0	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	555	641	569
Recorded Investment	555	641	569
Average Recorded Investment	562	369	498
Interest Income Recognized in Period	1	0	3
Cash Basis Interest Recognized in Period	1	0	3
Consumer
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Related Allowance	0	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized in Period	0	0	0
Cash Basis Interest Recognized in Period	0	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	0	0	0
Recorded Investment	0	0	0
Average Recorded Investment	0	0	0
Interest Income Recognized in Period	0	0	0
Cash Basis Interest Recognized in Period	$ 0	$ 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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Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

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Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
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-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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Amount of interest income on financing receivables, for which there is a related allowance for credit losses, recognized on a cash-basis method of accounting that the financing receivables were impaired.

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Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

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