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Basis of Accounting and Consolidation Basis of Accounting and Consolidation (ATM) (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Aug. 16, 2017
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At The Market Program [Member] |  |
Balance of Common Stock Available Under ATM | $ 16,530 | $ 20,000

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- Definition Value of common stock available under At-The-Market Program + References No definition available. + Details
Name: | fmbh_AvailableCommonStockUnderSalesAgencyAgreement
Namespace Prefix: | fmbh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_StatementClassOfStockAxis=fmbh_AtTheMarketProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |