v3.25.2

Employee Stock Ownership Plan (ESOP)- Activity in the ESOP (Details) - USD ($)	9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024
Employee Stock Ownership Plan (ESOP)
Shares allocated to participants	10,912	5,456
Unreleased shares	70,926	76,382
ESOP shares at end of plan year	81,838	81,838
Fair value of unreleased shares	$ 1,039,066	$ 1,031,157

The shares or units in an ESOP trust that have been assigned to individual participant accounts based on a known formula. IRS rules require allocations to be nondiscriminatory generally based on compensation, length of service, or a combination of both. For any particular participant such shares or units may be vested, unvested, or partially vested.

+ References

No definition available.

+ Details

Name:	merbc_EmployeeStockOwnershipPlanEsopNumberOfSharesAllocatedToParticipants
Namespace Prefix:	merbc_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Shares contributed to the ESOP that have not yet been released, committed to be released, or allocated to participant accounts.

+ References

No definition available.

+ Details

Name:	merbc_EmployeeStockOwnershipPlanEsopNumberOfUnreleasedShares
Namespace Prefix:	merbc_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Fair value of unearned shares held by the employee stock ownership plan (ESOP).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 40
-Section 50
-Paragraph 1
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480489/718-40-50-1

+ Details

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

- Definition

Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 718
-SubTopic 40
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480489/718-40-50-1

+ Details

Name:	us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Mercer Bancorp, Inc. 10-Q