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Investment Securities - Realized Gains and Losses From Sale of Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Realized Investment Gains (Losses) [Abstract]
Gross gains	$ 0	$ 0
Gross losses	$ (181)	$ 0

X

- Definition

Debt and equity securities available for sale realized gain.

+ References

No definition available.

+ Details

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X

- Definition

Debt and equity securities available for sale realized loss.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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