Loans and Allowance for Credit Losses - Amortized Cost Basis of Loans on Nonaccrual Status and Nonaccrual Loans Individually Evaluated (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	$ 19,213	$ 14,318
Financing Receivable, Nonaccrual	20,787	28,775
Construction and Land Development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	5	6
Financing Receivable, Nonaccrual	5	6
Agricultural Real Estate Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	1,097	2,213
Financing Receivable, Nonaccrual	1,097	2,213
1-4 Family Residential Properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	4,522	4,196
Financing Receivable, Nonaccrual	5,344	4,937
Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	9,785	4,901
Financing Receivable, Nonaccrual	9,785	7,716
Loans Secured by Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	15,409	11,316
Financing Receivable, Nonaccrual	16,231	14,872
Agricultural Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	190	1,371
Financing Receivable, Nonaccrual	190	11,521
Commercial and Industrial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	1,389	1,320
Financing Receivable, Nonaccrual	2,140	2,071
Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	150	311
Financing Receivable, Nonaccrual	150	311
All Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Nonaccrual, No Allowance	2,075	0
Financing Receivable, Nonaccrual	$ 2,076	$ 0

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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

+ Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amortized cost of financing receivable on nonaccrual status with no allowance for credit loss. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

Name:	us-gaap_FinancingReceivableNonaccrualNoAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost of financing receivable on nonaccrual status. Excludes net investment in lease.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 7
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph a
-Subparagraph (3)
-Publisher SEC

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph a
-Subparagraph (2)
-Publisher SEC

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-16

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fmbh_AgriculturalRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fmbh_ResidentialPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_RealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fmbh_AgriculturalLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: