Loans Receivable and Related Allowance for Loan Losses - Impaired Loans by Class (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Impaired Loans with Specific Allowance
Unpaid Principal Balance	$ 81	$ 76	$ 81	$ 76	$ 83
Recorded Investment	81	76	81	76	83
Related Allowance	7	8	7	8	7
Average Recorded Investment	83	76	82	478	319
Interest Income Recognized in Period	1	1	2	2	3
Cash Basis Interest Recognized in Period	1	1	2	2	3
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	528	2,227	528	2,227	2,119
Recorded Investment	416	1,941	416	1,941	1,833
Average Recorded Investment	826	1,972	1,161	1,499	1,642
Interest Income Recognized in Period	116	3	117	6	14
Cash Basis Interest Recognized in Period	116	3	117	6	14
Residential first mortgages
Impaired Loans with Specific Allowance
Unpaid Principal Balance	74	76	74	76	75
Recorded Investment	74	76	74	76	75
Related Allowance	7	8	7	8	7
Average Recorded Investment	75	76	75	96	88
Interest Income Recognized in Period	1	1	2	2	3
Cash Basis Interest Recognized in Period	1	1	2	2	3
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	446	478	446	478	461
Recorded Investment	334	366	334	366	350
Average Recorded Investment	339	369	343	246	289
Interest Income Recognized in Period	2	3	2	4	8
Cash Basis Interest Recognized in Period	2	3	2	4	8
Home equity loans and lines of credit
Impaired Loans with Specific Allowance
Unpaid Principal Balance	7	0	7	0	8
Recorded Investment	7	0	7	0	8
Related Allowance	0	0	0	0	0
Average Recorded Investment	8	0	7	0	2
Interest Income Recognized in Period	0	0	0	0	0
Cash Basis Interest Recognized in Period	0	0	0	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	0	0	0	0	0
Recorded Investment	0	0	0	0	0
Average Recorded Investment	0	0	0	0	0
Interest Income Recognized in Period	0	0	0	0	0
Cash Basis Interest Recognized in Period	0	0	0	0	0
Commercial real estate
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	0	0	0	0
Recorded Investment	0	0	0	0	0
Related Allowance	0	0	0	0	0
Average Recorded Investment	0	0	0	186	111
Interest Income Recognized in Period	0	0	0	0	0
Cash Basis Interest Recognized in Period	0	0	0	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	43	1,149	43	1,149	1,089
Recorded Investment	43	975	43	975	914
Average Recorded Investment	190	983	431	807	855
Interest Income Recognized in Period	73	0	73	1	3
Cash Basis Interest Recognized in Period	73	0	73	1	3
Commercial business
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	0	0	0	0
Recorded Investment	0	0	0	0	0
Related Allowance	0	0	0	0	0
Average Recorded Investment	0	0	0	196	118
Interest Income Recognized in Period	0	0	0	0	0
Cash Basis Interest Recognized in Period	0	0	0	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	39	600	39	600	569
Recorded Investment	39	600	39	600	569
Average Recorded Investment	297	620	387	446	498
Interest Income Recognized in Period	41	0	42	1	3
Cash Basis Interest Recognized in Period	41	0	42	1	3
Consumer
Impaired Loans with Specific Allowance
Unpaid Principal Balance	0	0	0	0	0
Recorded Investment	0	0	0	0	0
Related Allowance	0	0	0	0	0
Average Recorded Investment	0	0	0	0	0
Interest Income Recognized in Period	0	0	0	0	0
Cash Basis Interest Recognized in Period	0	0	0	0	0
Impaired Loans with No Specific Allowance
Unpaid Principal Balance	0	0	0	0	0
Recorded Investment	0	0	0	0	0
Average Recorded Investment	0	0	0	0	0
Interest Income Recognized in Period	0	0	0	0	0
Cash Basis Interest Recognized in Period	$ 0	$ 0	$ 0	$ 0	$ 0

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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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