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Investment Securities - Realized Gains and Losses From Sale of Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Realized Investment Gains (Losses) [Abstract]
Gross gains	$ 4	$ 531
Gross losses	$ 0	$ 0

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- Definition

Debt and equity securities available for sale realized gain.

+ References

No definition available.

+ Details

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- 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

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- References

No definition available.

+ Details

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