Loans Receivable - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 9,282	$ 8,523	$ 8,760	$ 8,060
Provision for loan losses	197	0	902	500
Charge-offs	(267)	(70)	(540)	(323)
Recoveries	123	155	213	371
Ending balance	9,335	8,608	9,335	8,608
Total ALLL	9,282	8,523	8,760	8,608	$ 9,335	$ 8,760
General reserve					8,694	8,563
Specific reserve					641	197
Total loans					845,797	786,141
Loans collectively evaluated					838,203	780,080
Loans individually evaluated					7,594	6,061
Real Estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total loans					720,618	703,259
Real Estate | Real estate loans | One-to-four family
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	3,050	3,071	3,061	2,892
Provision for loan losses	3	(263)	6	(111)
Charge-offs	(2)	0	(18)	0
Recoveries	2	100	4	127
Ending balance	3,053	2,908	3,053	2,908
Total ALLL	3,050	3,071	3,061	2,908	3,053	3,061
General reserve					3,013	3,014
Specific reserve					40	47
Total loans					336,739	355,391
Loans collectively evaluated					332,948	351,545
Loans individually evaluated					3,791	3,846
Real Estate | Real estate loans | Multi-family
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	841	511	648	370
Provision for loan losses	(31)	8	162	149
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending balance	810	519	810	519
Total ALLL	841	511	648	519	810	648
General reserve					809	647
Specific reserve					1	1
Total loans					85,229	73,767
Loans collectively evaluated					85,117	73,652
Loans individually evaluated					112	115
Real Estate | Commercial real estate
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	2,160	1,735	1,847	1,488
Provision for loan losses	(47)	(93)	266	154
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending balance	2,113	1,642	2,113	1,642
Total ALLL	2,160	1,735	1,847	1,642	2,113	1,847
General reserve					2,105	1,719
Specific reserve					8	128
Total loans					239,431	202,956
Loans collectively evaluated					237,430	201,885
Loans individually evaluated					2,001	1,071
Real Estate | Construction and land
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	514	683	648	585
Provision for loan losses	28	75	(107)	172
Charge-offs	0	0	0	0
Recoveries	0	0	1	1
Ending balance	542	758	542	758
Total ALLL	514	683	648	758	542	648
General reserve					541	647
Specific reserve					1	1
Total loans					59,219	71,145
Loans collectively evaluated					59,102	71,093
Loans individually evaluated					117	52
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total loans					108,747	66,579
Consumer | Home equity
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	642	818	787	794
Provision for loan losses	(81)	(71)	(242)	(103)
Charge-offs	0	0	0	(79)
Recoveries	7	16	23	151
Ending balance	568	763	568	763
Total ALLL	642	818	787	763	568	787
General reserve					563	779
Specific reserve					5	8
Total loans					38,744	38,473
Loans collectively evaluated					38,139	37,838
Loans individually evaluated					605	635
Consumer | Auto and other consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	1,332	523	712	361
Provision for loan losses	179	87	986	366
Charge-offs	(265)	(70)	(522)	(239)
Recoveries	114	39	184	91
Ending balance	1,360	579	1,360	579
Total ALLL	1,332	523	712	579	1,360	712
General reserve					1,334	703
Specific reserve					26	9
Total loans					70,003	28,106
Loans collectively evaluated					69,873	28,047
Loans individually evaluated					130	59
Commercial business loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	716	1,168	265	652
Provision for loan losses	(31)	(1,043)	419	(523)
Charge-offs	0	0	0	(5)
Recoveries	0	0	1	1
Ending balance	685	125	685	125
Total ALLL	716	1,168	265	125	685	265
General reserve					125	262
Specific reserve					560	3
Total loans					16,432	16,303
Loans collectively evaluated					15,594	16,020
Loans individually evaluated					838	283
Unallocated
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance	27	14	792	918
Provision for loan losses	177	1,300	(588)	396
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Ending balance	204	1,314	204	1,314
Total ALLL	$ 27	$ 14	$ 792	$ 1,314	204	792
General reserve					204	792
Specific reserve					0	0
Total loans					0	0
Loans collectively evaluated					0	0
Loans individually evaluated					$ 0	$ 0