Investment Securities - Realized Gains and Losses From Sale of Securities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Realized Investment Gains (Losses) [Abstract]
Gross gains	$ 6	$ 0
Gross losses	$ (52)	$ 0

Debt and equity securities available for sale realized gain.

No definition available.

Name:	fmbh_DebtAndEquitySecuritiesAvailableForSaleRealizedGain
Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration