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Derivatives - Summary of Hedged Instrument (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Dec. 31, 2021
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Hedged Instruments [Abstract] |  |
Carrying Amount of the Hedged Asset | $ 12,295 | $ 13,234
Cumulative Amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Asset | $ (1,153) | $ 667

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- Definition Hedged Instruments [Abstract] + References No definition available. + Details
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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 https://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959 + Details
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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 https://asc.fasb.org/extlink&oid=125515794&loc=SL109999712-113959 + Details
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Namespace Prefix: | us-gaap_
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