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Cash and Due from Banks - Additional Information (Details) | 12 Months Ended
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Dec. 31, 2023 USD ($)
Cash and Cash Equivalents [Abstract] |
Cash uninsured amount | $ 14,400,000
Loss on cash uninsured | $ 0

X
- Definition Loss on cash uninsured. + References No definition available. + Details
Name: | fmbh_LossOnCashUninsured
Namespace Prefix: | fmbh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_CashAndCashEquivalentsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of cash as of the balance sheet date that is not insured by the Federal Deposit Insurance Corporation. + References No definition available. + Details
Name: | us-gaap_CashUninsuredAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant