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Derivatives Hedged Instrument (Details) $ in Thousands | Sep. 30, 2019 USD ($)
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Hedged Instruments [Abstract] |
Hedged Asset, Fair Value Hedge | $ 9,126
Hedged Asset, Fair Value Hedge, Cumulative Increase (Decrease) | $ 605

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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 http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959 + Details
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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=109980867&loc=SL109999712-113959 + Details
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Namespace Prefix: | us-gaap_
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