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Deposits - Additional Information (Details) - USD ($) $ in Millions | Dec. 31, 2022 | Dec. 31, 2021
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Demand deposits: |  |
Total of Public entitiy balances | $ 319.4 | $ 291.4

X
- Definition Total balance of Public entities with large balances + References No definition available. + Details
Name: | fmbh_TotalOfPublicEntitiyBalances
Namespace Prefix: | fmbh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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