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Deposits - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Demand deposits:
Total of Public entitiy balances	$ 227.6	$ 258.4

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- Definition

Total balance of Public entities with large balances

+ References

No definition available.

+ Details

Name:	fmbh_TotalOfPublicEntitiyBalances
Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- References

No definition available.

+ Details

Name:	us-gaap_DepositsByTypeAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration