Loans - Activity in the allowance for credit losses by portfolio segment (Details) - USD ($)	12 Months Ended
Jun. 30, 2025	Jun. 30, 2024
Activity in the allowance for credit losses - loans by loan class
Beginning Balance	$ 1,797,116	$ 2,159,000
Recoveries	5,000	3,000
Provision for (Recovery of) Credit Losses-Loans	(94,000)	(190,000)
Ending Balance	1,708,269	1,797,116
Cumulative effect of accounting change
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		(175,000)
Consumer
Activity in the allowance for credit losses - loans by loan class
Beginning Balance	5,000	2,000
Recoveries	5,000	3,000
Provision for (Recovery of) Credit Losses-Loans	(1,000)	(11,000)
Ending Balance	9,000	5,000
Consumer \| Cumulative effect of accounting change
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		11,000
Commercial real estate \| Commercial real estate
Activity in the allowance for credit losses - loans by loan class
Beginning Balance	259,000	1,196,000
Provision for (Recovery of) Credit Losses-Loans	131,000	(119,000)
Ending Balance	390,000	259,000
Commercial real estate \| Commercial real estate \| Cumulative effect of accounting change
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		(818,000)
Commercial \| Commercial and industrial
Activity in the allowance for credit losses - loans by loan class
Beginning Balance	16,000	18,000
Provision for (Recovery of) Credit Losses-Loans	(5,000)	(7,000)
Ending Balance	11,000	16,000
Commercial \| Commercial and industrial \| Cumulative effect of accounting change
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		5,000
Commercial \| Construction
Activity in the allowance for credit losses - loans by loan class
Beginning Balance	28,000	6,000
Provision for (Recovery of) Credit Losses-Loans	(24,000)	20,000
Ending Balance	4,000	28,000
Commercial \| Construction \| Cumulative effect of accounting change
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		2,000
Residential \| One-to-four-family residential
Activity in the allowance for credit losses - loans by loan class
Beginning Balance	1,314,000	207,000
Provision for (Recovery of) Credit Losses-Loans	(191,000)	(30,000)
Ending Balance	1,123,000	1,314,000
Residential \| One-to-four-family residential \| Cumulative effect of accounting change
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		1,137,000
Residential \| Multi-family real estate
Activity in the allowance for credit losses - loans by loan class
Beginning Balance	175,000	365,000
Provision for (Recovery of) Credit Losses-Loans	(4,000)	(43,000)
Ending Balance	$ 171,000	175,000
Residential \| Multi-family real estate \| Cumulative effect of accounting change
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		(147,000)
Unallocated
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		365,000
Unallocated \| Cumulative effect of accounting change
Activity in the allowance for credit losses - loans by loan class
Beginning Balance		$ (365,000)

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

+ Details

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

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

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

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mara_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=mara_OneToFourFamilyResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=srt_MultifamilyMember
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: