Loans Receivable and Allowance for Loan and Lease Losses Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - Beginning Balance	$ 5,126	$ 4,747	$ 5,060	$ 5,232	$ 5,232
Charge-offs	70	54	211	408	529
Recoveries	8	55	91	288	328
Provision for loan losses	6	(128)	130	(492)	29
Allowance for loan losses - Ending Balance	5,070	4,620	5,070	4,620	5,060
Allowance for Loan Losses, Collectively Evaluated for Impairment	4,726	4,281	4,726	4,281	4,699
Allowance for Loan Losses, Individually Evaluated for Impairment	344	339	344	339	361
Loans, gross	496,708	532,818	496,708	532,818	545,552
Loans Receivable, Collectively Evaluated for Impairment	483,726	515,672	483,726	515,672	530,859
Loans Receivable, Individually Evaluated for Impairment	12,982	17,146	12,982	17,146	14,693
One-to-four family
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - Beginning Balance	1,179	1,654	1,663	1,697	1,697
Charge-offs	0	4	93	66	106
Recoveries	0	0	0	0	0
Provision for loan losses	(52)	(33)	(443)	(14)	72
Allowance for loan losses - Ending Balance	1,127	1,617	1,127	1,617	1,663
Allowance for Loan Losses, Collectively Evaluated for Impairment	976	1,369	976	1,369	1,357
Allowance for Loan Losses, Individually Evaluated for Impairment	151	248	151	248	306
Loans, gross	125,678	138,620	125,678	138,620	134,927
Loans Receivable, Collectively Evaluated for Impairment	119,226	133,097	119,226	133,097	129,420
Loans Receivable, Individually Evaluated for Impairment	6,452	5,523	6,452	5,523	5,507
Multifamily
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - Beginning Balance	1,369	672	1,213	622	622
Charge-offs	36	37	42	288	338
Recoveries	0	0	0	0	0
Provision for loan losses	(25)	54	137	355	929
Allowance for loan losses - Ending Balance	1,308	689	1,308	689	1,213
Allowance for Loan Losses, Collectively Evaluated for Impairment	1,172	689	1,172	689	1,207
Allowance for Loan Losses, Individually Evaluated for Impairment	136	0	136	0	6
Loans, gross	75,805	71,924	75,805	71,924	88,750
Loans Receivable, Collectively Evaluated for Impairment	74,160	70,321	74,160	70,321	87,148
Loans Receivable, Individually Evaluated for Impairment	1,645	1,603	1,645	1,603	1,602
Commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - Beginning Balance	1,692	1,743	1,496	1,808	1,808
Charge-offs	0	0	0	0	0
Recoveries	5	4	15	14	20
Provision for loan losses	(25)	(180)	161	(255)	(332)
Allowance for loan losses - Ending Balance	1,672	1,567	1,672	1,567	1,496
Allowance for Loan Losses, Collectively Evaluated for Impairment	1,672	1,537	1,672	1,537	1,490
Allowance for Loan Losses, Individually Evaluated for Impairment	0	30	0	30	6
Loans, gross	219,939	250,810	219,939	250,810	242,818
Loans Receivable, Collectively Evaluated for Impairment	219,431	246,339	219,431	246,339	239,323
Loans Receivable, Individually Evaluated for Impairment	508	4,471	508	4,471	3,495
Construction
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - Beginning Balance	0	50	106	62	62
Charge-offs	0	0	0	0	0
Recoveries	0	0	0	0	0
Provision for loan losses	0	70	(106)	58	44
Allowance for loan losses - Ending Balance	0	120	0	120	106
Allowance for Loan Losses, Collectively Evaluated for Impairment	0	120	0	120	106
Allowance for Loan Losses, Individually Evaluated for Impairment	0	0	0	0	0
Loans, gross	0	4,962	0	4,962	4,949
Loans Receivable, Collectively Evaluated for Impairment	0	4,962	0	4,962	4,949
Loans Receivable, Individually Evaluated for Impairment	0	0	0	0	0
Business
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - Beginning Balance	832	577	573	1,022	1,022
Charge-offs	27	0	47	0	0
Recoveries	3	51	72	270	304
Provision for loan losses	(31)	(29)	179	(693)	(753)
Allowance for loan losses - Ending Balance	777	599	777	599	573
Allowance for Loan Losses, Collectively Evaluated for Impairment	720	541	720	541	532
Allowance for Loan Losses, Individually Evaluated for Impairment	57	58	57	58	41
Loans, gross	69,357	66,200	69,357	66,200	65,114
Loans Receivable, Collectively Evaluated for Impairment	64,980	60,654	64,980	60,654	61,027
Loans Receivable, Individually Evaluated for Impairment	4,377	5,546	4,377	5,546	4,087
Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - Beginning Balance	20	2	9	21	21
Charge-offs	7	13	29	54	85
Recoveries	0	0	4	4	4
Provision for loan losses	8	39	37	57	69
Allowance for loan losses - Ending Balance	21	28	21	28	9
Allowance for Loan Losses, Collectively Evaluated for Impairment	21	25	21	25	7
Allowance for Loan Losses, Individually Evaluated for Impairment	0	3	0	3	2
Loans, gross	5,929	302	5,929	302	8,994
Loans Receivable, Collectively Evaluated for Impairment	5,929	299	5,929	299	8,992
Loans Receivable, Individually Evaluated for Impairment	0	3	0	3	2
Unallocated
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan losses - Beginning Balance	34	49	0	0	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for loan losses	131	(49)	165	0
Allowance for loan losses - Ending Balance	165	0	165	0	$ 0
Allowance for Loan Losses, Collectively Evaluated for Impairment	165	0	165	0
Allowance for Loan Losses, Individually Evaluated for Impairment	0	0	0	0
Loans, gross	0	0	0	0
Loans Receivable, Collectively Evaluated for Impairment	0	0	0	0
Loans Receivable, Individually Evaluated for Impairment	$ 0	$ 0	$ 0	$ 0

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

Name:	us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetOfDeferredIncome
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_ProvisionForLoanAndLeaseLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: