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Investment Securities - Schedule of Proceeds From Sales of Available for Sale Investment Securities, Realized Gains and Losses and Income Tax Expense (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Realized Investment Gains (Losses) [Abstract] |  |  |
Proceeds from sales | $ 8,864 | $ 60,900 | $ 13,152
Gross gains | 1,132 | 875 | 941
Gross losses | (26) | (73) | (40)
Income tax expense | $ 321 | $ 233 | $ 261

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- Definition This item represents the net income tax expense (benefit) on the gain or loss resulting from the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | fmbh_AvailableForSaleSecuritiesIncomeTaxExpense
Namespace Prefix: | fmbh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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

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- Definition The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26853-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (a),(b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3179-108585 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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