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Commitments and Contingent Liabilities - Additional Information (Details) | 12 Months Ended
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Dec. 31, 2023
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

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- 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: |