Loans - Activity in the allowance for loan losses by portfolio segment (Details) - USD ($)	3 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2022
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year	$ 2,195,050	$ 2,186,182
Charge-offs	(136,753)
Recoveries	572	1,124
Balance at end of year	2,058,869	2,187,306
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Allowance	2,058,869	2,187,306	$ 2,195,050
Collectively evaluated for impairment	2,058,869		2,195,050
Loans and Leases Receivable, Gross	195,543,646		187,900,474
Loans - Individually evaluated for impairment	156,056		193,385
Loans - Collectively evaluated for impairment	195,387,590		187,707,089
Commercial real estate [Member] \| Commercial real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year	1,591,644	1,036,301
Charge-offs	(136,753)
Provisions	109,130	187,549
Balance at end of year	1,564,021	1,223,850
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Allowance	1,564,021	1,223,850	1,591,644
Collectively evaluated for impairment	1,564,021		1,591,644
Loans and Leases Receivable, Gross	84,854,449		80,603,153
Loans - Individually evaluated for impairment	23,549
Loans - Collectively evaluated for impairment	84,830,900		80,603,153
Commercial [Member]
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Gross	101,751,199		99,964,364
Commercial [Member] \| Commercial real estate
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Gross	84,854,449		80,603,153
Commercial [Member] \| Commercial and industrial
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year	32,701	157,533
Provisions	(5,303)	(89,036)
Balance at end of year	27,398	68,497
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Allowance	27,398	68,497	32,701
Collectively evaluated for impairment	27,398		32,701
Loans and Leases Receivable, Gross	9,209,475		8,778,723
Loans - Collectively evaluated for impairment	9,209,475		8,778,723
Commercial [Member] \| Construction
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year	55,029	59,649
Provisions	(30,238)	(9,003)
Balance at end of year	24,791	50,646
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Allowance	24,791	50,646	55,029
Collectively evaluated for impairment	24,791		55,029
Loans and Leases Receivable, Gross	7,687,275		10,582,488
Loans - Collectively evaluated for impairment	7,687,275		10,582,488
Residential [Member] \| One-to-four-family residential
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year	263,951	409,395
Provisions	(41,667)	(53,138)
Balance at end of year	222,284	356,257
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Allowance	222,284	356,257	263,951
Collectively evaluated for impairment	222,284		263,951
Loans and Leases Receivable, Gross	56,367,980		51,890,948
Loans - Individually evaluated for impairment	132,507		193,385
Loans - Collectively evaluated for impairment	56,235,473		51,697,563
Residential [Member] \| Multi-family real estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year	233,371	134,216
Provisions	(17,238)	(47,734)
Balance at end of year	216,133	86,482
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Allowance	216,133	86,482	233,371
Collectively evaluated for impairment	216,133		233,371
Loans and Leases Receivable, Gross	35,287,008		33,944,903
Loans - Collectively evaluated for impairment	35,287,008		33,944,903
Consumer Loans [Member] \| Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year	601	4,896
Recoveries	572	1,124
Provisions	(553)	(102)
Balance at end of year	620	5,918
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Allowance	620	5,918	601
Collectively evaluated for impairment	620		601
Loans and Leases Receivable, Gross	2,137,459		2,100,259
Loans - Collectively evaluated for impairment	2,137,459		2,100,259
Unallocated
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of year	17,753	384,192
Provisions	(14,131)	11,464
Balance at end of year	3,622	395,656
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Loans and Leases Receivable, Allowance	3,622	$ 395,656	17,753
Collectively evaluated for impairment	$ 3,622		$ 17,753

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

Name:	us-gaap_AllowanceForLoanAndLeaseLossRecoveryOfBadDebts
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_AllowanceForLoanAndLeaseLossesRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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

- References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ 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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ 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/2009/role/commonPracticeRef
-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=122038336&loc=d3e74567-122707

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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=121614798&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=124259787&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=123577603&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_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_ConsumerPortfolioSegmentMember
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_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: