Number of branch offices in which the bank conducts business.

No definition available.

Name:	mara_NumberOfBranchOffices
Namespace Prefix:	mara_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of common stock that will be offered to the Bank's eligible members, the ESOP and certain other persons.

+ References

No definition available.

+ Details

Name:	mara_PercentageOfCommonStockOfferingToBankEligibleMembersEsopAndCertainOtherPersons
Namespace Prefix:	mara_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of reorganization costs incurred during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	mara_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the threshold percentage of common stock permitted to be subscribed under the ESOP by the board of directors.

+ References

No definition available.

+ Details

Name:	mara_ThresholdPercentageOfCommonStockPermittedForSubscriptionUnderESOP
Namespace Prefix:	mara_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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

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

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_OwnershipAxis=mara_MarathonBancorpInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Basis of Presentation (Details)			6 Months Ended
Basis of Presentation (Details)	Dec. 31, 2020 USD ($)	Dec. 09, 2020 $ / shares	Dec. 31, 2020 item
Marathon Bancorp, Inc.
Class of Stock [Line Items]
Number of branch offices \| item			3
Percentage of common stock to be outstanding		45.00%
Reorganization costs \| $	$ 324,000
Marathon Mutual Holding Company
Class of Stock [Line Items]
Share Price \| $ / shares		$ 10.00
Percentage of common stock permitted to be subscribed under ESOP		3.92%
Marathon Mutual Holding Company \| Marathon Bancorp, Inc.
Class of Stock [Line Items]
Ownership percentage by parent		55.00%