Loans Receivable - Reconciliation of Past Due Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2017
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	$ 2,331	$ 2,957	$ 686
Current	867,399	783,184	733,311
Total loans	869,730	786,141	733,997
Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Total loans	767,600	742,900	697,500
Residential segment
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	709	1,741	260
Current	725,137	701,518	659,752
Total loans	725,846	703,259	660,012
Residential segment | Commercial real estate
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	91	0
Current	253,235	202,865	202,038
Total loans	253,235	202,956	202,038
Residential segment | Construction and land
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	80	1,206	54
Current	54,022	69,939	71,576
Total loans	54,102	71,145	71,630
Consumer
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	699	568	426
Current	124,287	66,011	56,486
Total loans	124,986	66,579	56,912
Consumer | Home equity
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	136	461	325
Current	37,493	38,012	35,544
Total loans	37,629	38,473	35,869
Consumer | Auto and other consumer
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	563	107	101
Current	86,794	27,999	20,942
Total loans	87,357	28,106	21,043
Commercial business loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	923	648	0
Current	17,975	15,655	17,073
Total loans	18,898	16,303	17,073
30 to 59 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	1,815	2,599	49
30 to 59 Days Past Due | Residential segment
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	324	1,491	0
30 to 59 Days Past Due | Residential segment | Commercial real estate
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	91	0
30 to 59 Days Past Due | Residential segment | Construction and land
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	35	1,187	0
30 to 59 Days Past Due | Consumer
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	568	460	49
30 to 59 Days Past Due | Consumer | Home equity
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	97	383	21
30 to 59 Days Past Due | Consumer | Auto and other consumer
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	471	77	28
30 to 59 Days Past Due | Commercial business loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	923	648	0
60 to 89 Days Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	312	108	607
60 to 89 Days Past Due | Residential segment
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	190	0	240
60 to 89 Days Past Due | Residential segment | Commercial real estate
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	0	0
60 to 89 Days Past Due | Residential segment | Construction and land
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	14	0	34
60 to 89 Days Past Due | Consumer
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	122	108	367
60 to 89 Days Past Due | Consumer | Home equity
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	30	78	294
60 to 89 Days Past Due | Consumer | Auto and other consumer
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	92	30	73
60 to 89 Days Past Due | Commercial business loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	0	0
90 Days or More Past Due
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	204	250	30
90 Days or More Past Due | Residential segment
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	195	250	20
90 Days or More Past Due | Residential segment | Commercial real estate
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	0	0
90 Days or More Past Due | Residential segment | Construction and land
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	31	19	20
90 Days or More Past Due | Consumer
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	9	0	10
90 Days or More Past Due | Consumer | Home equity
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	9	0	10
90 Days or More Past Due | Consumer | Auto and other consumer
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	0	0
90 Days or More Past Due | Commercial business loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	0	0
One- to four-family | Residential segment | Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	629	444	206
Current	335,549	354,947	328,037
Total loans	336,178	355,391	328,243
One- to four-family | 30 to 59 Days Past Due | Residential segment | Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	289	213	0
One- to four-family | 60 to 89 Days Past Due | Residential segment | Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	176	0	206
One- to four-family | 90 Days or More Past Due | Residential segment | Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	164	231	0
Multi-family | Residential segment | Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	0	0
Current	82,331	73,767	58,101
Total loans	82,331	73,767	58,101
Multi-family | 30 to 59 Days Past Due | Residential segment | Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	0	0
Multi-family | 60 to 89 Days Past Due | Residential segment | Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	0	0	0
Multi-family | 90 Days or More Past Due | Residential segment | Real estate loans
Financing Receivable, Recorded Investment, Aging [Abstract]
Past due loans	$ 0	$ 0	$ 0