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Commitments - Additional Information (Details)	3 Months Ended
	Mar. 31, 2025
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Number of expected days to fund commitments	90 days
Financial Standby Letter of Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Period when letters of credit expire	one year or less

X

- Definition

Number of expected days until commitments to originate credit are funded.

+ References

No definition available.

+ Details

Name:	fmbh_NumberOfExpectedDaysToFundCommitments
Namespace Prefix:	fmbh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Period after which standby letters of credit generally expire

+ References

No definition available.

+ Details

Name:	fmbh_PeriodWhenLettersOfCreditExpire
Namespace Prefix:	fmbh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

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

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_FinancialStandbyLetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: