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v3.20.1

Basis of Accounting and Consolidation Basis of Accounting and Consolidation (Acquisitions) (Details) $ in Millions | Apr. 21, 2020 USD ($)
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Subsequent Event [Member] |
Subsequent Event [Line Items] |
LoansAcquired | $ 183

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- Definition Loans Acquired + References No definition available. + Details
Name: | fmbh_LoansAcquired
Namespace Prefix: | fmbh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |