Deposits - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Demand deposits:
Total of Public entitiy balances	$ 319.4	$ 291.4

Total balance of Public entities with large balances

No definition available.

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