Commitments and Contingent Liabilities - Schedule of Off-balance Sheet Financial Instruments Whose Contract Amounts Represent Credit Risk (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Commercial real estate	$ 147,702	$ 118,190
Commercial operating	655,676	529,035
Home equity	63,570	59,422
Other	307,030	293,339
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Asset	1,173,978	999,986
Financial Standby Letter of Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value Disclosure, Off-balance Sheet Risks, Face Amount, Asset	$ 10,162	$ 14,403

- Definition

The face amount of Commercial Operating financial assets, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

No definition available.

+ Details

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- Definition

The face amount of Commercial Real Estate financial assets, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

No definition available.

+ Details

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- Definition

The face amount of Home Equity financial assets, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

No definition available.

+ Details

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Namespace Prefix:	fmbh_
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- Definition

The face amount of Other financial assets, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
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- Definition

The face amount of financial assets, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

No definition available.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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- Details

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