Loans Receivable - Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended														6 Months Ended		12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2017
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				$ 8,760		$ 8,523				$ 7,239				$ 7,111	$ 8,523	$ 7,239	$ 8,760		$ 7,239	$ 7,111
Provision for loan losses	$ 272	$ 197	$ 395	310	$ 200	0	$ 285	$ 215	$ 410	350	$ 233	$ 0	$ 0	0	200	760	1,174	$ 200	1,260	233
Charge-offs																	(656)	(206)	(338)	(366)
Recoveries																	255	243	362	261
Ending balance	9,533				8,760		8,523				7,239				8,760		9,533	8,760	8,523	7,239
Total ALLL	9,533			8,760	8,760	8,523	8,523			7,239	7,239			7,111	8,523	7,239	8,760	8,760	7,239	7,111	$ 9,533	$ 8,760	$ 8,523
General reserve																					9,257	8,563	8,171
Specific reserve																					276	197	352
Total loans																					869,730	786,141	733,997
General reserves																					863,172	780,080	726,609
Specific reserves																					6,558	6,061	7,388
Residential segment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total loans																					725,846	703,259	660,012
Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total loans																					124,986	66,579	56,912
Commercial business
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				265		1,168				335				251	1,168	335	265		335	251
Provision for loan losses																	68	(904)	836	49
Charge-offs																	0	0	(5)	(7)
Recoveries																	1	1	2	42
Ending balance	334				265		1,168				335				265		334	265	1,168	335
Total ALLL	334			265	265	1,168	1,168			335	335			251	1,168	335	265	265	335	251	334	265	1,168
General reserve																					168	262	961
Specific reserve																					166	3	207
Total loans																					18,898	16,303	17,073
General reserves																					18,453	16,020	16,784
Specific reserves																					445	283	289
Unallocated
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				792		14				561				759	14	561	792		561	759
Provision for loan losses																	(617)	778	(547)	(198)
Charge-offs																	0	0	0	0
Recoveries																	0	0	0	0
Ending balance	175				792		14				561				792		175	792	14	561
Total ALLL	175			792	792	14	14			561	561			759	14	561	792	792	561	759	175	792	14
General reserve																					175	792	14
Specific reserve																					0	0	0
Total loans																					0	0	0
General reserves																					0	0	0
Specific reserves																					0	0	0
Real estate loans
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Total loans																					767,600	742,900	697,500
Real estate loans | One- to four-family | Residential segment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				3,061		3,071				2,992				3,143	3,071	2,992	3,061		2,992	3,143
Provision for loan losses																	249	(112)	(34)	(140)
Charge-offs																	(18)	0	0	(75)
Recoveries																	5	102	113	64
Ending balance	3,297				3,061		3,071				2,992				3,061		3,297	3,061	3,071	2,992
Total ALLL	3,297			3,061	3,061	3,071	3,071			2,992	2,992			3,143	3,071	2,992	3,061	3,061	2,992	3,143	3,297	3,061	3,071
General reserve																					3,262	3,014	2,988
Specific reserve																					35	47	83
Total loans																					336,178	355,391	328,243
General reserves																					333,062	351,545	323,592
Specific reserves																					3,116	3,846	4,651
Real estate loans | Multi-family | Residential segment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				648		511				341				251	511	341	648		341	251
Provision for loan losses																	114	137	170	90
Charge-offs																	0	0	0	0
Recoveries																	0	0	0	0
Ending balance	762				648		511				341				648		762	648	511	341
Total ALLL	762			648	648	511	511			341	341			251	511	341	648	648	341	251	762	648	511
General reserve																					761	647	510
Specific reserve																					1	1	1
Total loans																					82,331	73,767	58,101
General reserves																					82,221	73,652	57,983
Specific reserves																					110	115	118
Commercial real estate | Residential segment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				1,847		1,735				1,268				998	1,735	1,268	1,847		1,268	998
Provision for loan losses																	442	112	467	288
Charge-offs																	0	0	0	(18)
Recoveries																	0	0	0	0
Ending balance	2,289				1,847		1,735				1,268				1,847		2,289	1,847	1,735	1,268
Total ALLL	2,289			1,847	1,847	1,735	1,735			1,268	1,268			998	1,735	1,268	1,847	1,847	1,268	998	2,289	1,847	1,735
General reserve																					2,281	1,719	1,718
Specific reserve																					8	128	17
Total loans																					253,235	202,956	202,038
General reserves																					251,263	201,885	200,467
Specific reserves																					1,972	1,071	1,571
Construction and land | Residential segment
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				648		683				599				336	683	599	648		599	336
Provision for loan losses																	(65)	(36)	82	247
Charge-offs																	0	0	0	(17)
Recoveries																	2	1	2	33
Ending balance	585				648		683				599				648		585	648	683	599
Total ALLL	585			648	648	683	683			599	599			336	683	599	648	648	599	336	585	648	683
General reserve																					584	647	682
Specific reserve																					1	1	1
Total loans																					54,102	71,145	71,630
General reserves																					54,058	71,093	71,602
Specific reserves																					44	52	28
Home equity | Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				787		818				833				1,052	818	833	787		833	1,052
Provision for loan losses																	(332)	(6)	(90)	(205)
Charge-offs																	0	(47)	(81)	(77)
Recoveries																	25	22	156	63
Ending balance	480				787		818				833				787		480	787	818	833
Total ALLL	480			787	787	818	818			833	833			1,052	818	833	787	787	833	1,052	480	787	818
General reserve																					474	779	797
Specific reserve																					6	8	21
Total loans																					37,629	38,473	35,869
General reserves																					37,002	37,838	35,160
Specific reserves																					627	635	709
Auto and other consumer | Consumer
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning balance				712		523				310				321	523	310	712		310	321
Provision for loan losses																	1,315	231	376	102
Charge-offs																	(638)	(159)	(252)	(172)
Recoveries																	222	117	89	59
Ending balance	1,611				712		523				310				712		1,611	712	523	310
Total ALLL	$ 1,611			$ 712	$ 712	$ 523	$ 523			$ 310	$ 310			$ 321	$ 523	$ 310	$ 712	$ 712	$ 310	$ 321	1,611	712	523
General reserve																					1,552	703	501
Specific reserve																					59	9	22
Total loans																					87,357	28,106	21,043
General reserves																					87,113	28,047	21,021
Specific reserves																					$ 244	$ 59	$ 22