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Investment Securities - Realized Gains and Losses From Sale of Securities (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2022 | Mar. 31, 2021
Realized Investment Gains (Losses) [Abstract] |  |
Gross gains | $ 0 | $ 4
Gross losses | $ 0 | $ 0

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- Definition Debt and equity securities available for sale realized gain. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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- Definition Debt and equity securities available for sale realized loss. + References No definition available. + Details
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Namespace Prefix: | fmbh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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