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Derivatives - Summary of Hedged Instrument (Details) $ in Thousands | Sep. 30, 2021 USD ($)
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Hedged Instruments [Abstract] |
Carrying Amount of the Hedged Asset | $ 13,192
Cumulative Amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Asset | $ 864

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- Definition Hedged Instruments [Abstract] + References No definition available. + Details
Name: | fmbh_HedgedInstrumentsAbstract
Namespace Prefix: | fmbh_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of asset hedged in fair value hedging relationship. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4EE -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4EE -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant