The useful life of the loan receivable.

No definition available.

Name:	fmbh_AmortizationPeriodOfLoans
Namespace Prefix:	fmbh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The period of time when the final, large payment is due on a real estate loan.

+ References

No definition available.

+ Details

Name:	fmbh_BalloonPeriod
Namespace Prefix:	fmbh_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Definition

The ratio used to analyze the amount of debt that can be supported by the cash flow generated from the property. The calculation is the net income generated by the property divided by the commercial mortgage payment.

+ References

No definition available.

+ Details

Name:	fmbh_DebtCoverageRatio
Namespace Prefix:	fmbh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings and are still accruing interest.

+ References

No definition available.

+ Details

Name:	fmbh_FinancingReceivableModificationsPerformingRecordedInvestment
Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of the mortgage loan as a percentage of the total appraised value of the real property. Loan-to-value is one of the key risk factors that lenders assess when qualifying borrowers for a mortgage.

+ References

No definition available.

+ Details

Name:	fmbh_LoanToValueRatio
Namespace Prefix:	fmbh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Loans and leases receivable, interest receivable.

+ References

No definition available.

+ Details

Name:	fmbh_LoansAndLeasesReceivableInterestReceivable
Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The allowance for loan losses related to financing receivables that have been modified by troubled debt restructurings.

+ References

No definition available.

+ Details

Name:	fmbh_LoansReceivableModificationsAllowance
Namespace Prefix:	fmbh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Specifies the time period covered by the loan receivable.

+ References

No definition available.

+ Details

Name:	fmbh_LoansReceivableTimePeriod
Namespace Prefix:	fmbh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum value of loans individually measured for impairment.

+ References

No definition available.

+ Details

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

- Definition

Number of alternatives for measuring impaired loans receivable.

+ References

No definition available.

+ Details

Name:	fmbh_NumberOfAlternativesForMeasuringImpairedLoansReceivable
Namespace Prefix:	fmbh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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

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

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

+ Details

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

- Definition

Threshold period for when financing receivable is considered past due to write off as uncollectible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919253-210447

+ Details

Name:	us-gaap_FinancingReceivableThresholdPeriodPastDueWriteoff
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

The increase (decrease) during the reporting period of the sum of amounts due within one year (or one business cycle) from customers for the credit sale of goods and services; and from note holders for outstanding loans.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInAccountsAndNotesReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4428-111522

+ Details

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

- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18794-111554

+ Details

Name:	us-gaap_LoansAndLeasesReceivableImpairedInterestLostOnNonaccrualLoans
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL49126937-111524

+ Details

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

- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5162-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=121593590&loc=d3e4437-111522

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivablesEqualToGreaterThan90DaysPastDueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RestatementAxis=srt_RestatementAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredWithCreditDeteriorationMember
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:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbh_MotelsAndHotelsLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbh_NonResidentialBuildingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbh_ResidentialBuildingsAndDwellingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbh_OtherGamblingIndustriesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_RealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbh_UnsecuredOpenEndLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbh_OtherSecuredLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fmbh_AgriculturalPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fmbh_CornAndOtherGrainFarmingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fmbh_CommercialAndIndustrialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans and Allowance for Credit Losses - Additional Information (Details)	6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) alternative Loan_modified	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) Loan_modified	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest on loans	$ 14,800,000		$ 15,900,000
Minimum value of loans individually measured for impairment	$ 250,000
Number of alternatives for measuring impaired loans receivable \| alternative	3
Allowance for credit losses	$ 54,597,000	$ 38,381,000	41,910,000	$ 55,418,000	$ 32,876,000		$ 26,911,000
Financing receivable modifications performing recorded investment	3,560,000	2,600,000	4,373,000
Interest lost on nonaccrual loans	319,000	1,044,000
Troubled debt restructurings balance	8,210,000		9,502,000
Loans receivable, modifications, allowance	2,545,000		1,016,000
Real estate acquired through foreclosure	7,238,000		2,489,000
Mortgage loans secured by real estate In foreclosure	$ 471,000		$ 713,000
Financial Asset, Equal to or Greater than 90 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings balance \| Loan_modified	0		0
Accounting Standards Update 2016-13
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						$ 28,583,000
Accounting Standards Update 2016-13 \| Impact of ASU
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						1,672,000
Accounting Standards Update 2016-13 \| Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						1,198,000
Accounting Standards Update 2016-13 \| Financial Asset Acquired with Credit Deterioration \| Impact of ASU
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						833,000
Commercial Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Debt coverage ratio	1.20x
Amortization period of loans	20 years
Commercial Real Estate \| Accounting Standards Update 2016-13
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						11,739,000
Commercial Real Estate \| Accounting Standards Update 2016-13 \| Impact of ASU
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						541,000
Commercial Real Estate \| Accounting Standards Update 2016-13 \| Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						818,000
Commercial Real Estate \| Accounting Standards Update 2016-13 \| Financial Asset Acquired with Credit Deterioration \| Impact of ASU
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						459,000
Commercial Real Estate \| Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maximum loan-to-value ratio (in hundredths)	65.00%
Commercial Real Estate \| Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maximum loan-to-value ratio (in hundredths)	80.00%
1-4 Family Residential Properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period when loans charged-down	180 days
1-4 Family Residential Properties \| Accounting Standards Update 2016-13
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						2,142,000
1-4 Family Residential Properties \| Accounting Standards Update 2016-13 \| Impact of ASU
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						756,000
1-4 Family Residential Properties \| Accounting Standards Update 2016-13 \| Financial Asset Acquired with Credit Deterioration
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						48,000
1-4 Family Residential Properties \| Accounting Standards Update 2016-13 \| Financial Asset Acquired with Credit Deterioration \| Impact of ASU
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses						$ 42,000
Motels and Hotels Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 130,600,000
Non-residential Buildings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	554,500,000
Residential Buildings and Dwellings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	407,100,000
Other Gambling Industries
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	58,900,000
Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable modifications performing recorded investment	3,484,000		$ 4,313,000
Troubled debt restructurings balance	$ 5,537,000		6,773,000
Unsecured Open-end Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period when loans charged-down	180 days
Other Secured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period when loans charged-down	120 days
Agricultural
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 400,500,000		391,700,000
Increase (decrease) in accounts and notes receivable	$ 8,800,000
Maximum loan-to-value ratio (in hundredths)	65.00%
Amortization period of loans	25 years
Loans receivable, time period	1 year
Agricultural \| Corn and Other Grain Farming
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 278,600,000		308,200,000
Increase (decrease) in accounts and notes receivable	(29,600,000)
Agricultural \| Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses	$ 1,665,000	1,634,000	1,084,000	1,628,000	1,335,000		1,093,000
Commercial and Industrial Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maximum loan-to-value ratio (in hundredths)	80.00%
Amortization period of loans	7 years
Loans receivable, time period	1 year
Allowance for credit losses	$ 19,842,000	$ 12,360,000	13,485,000	$ 18,954,000	$ 11,294,000		$ 9,273,000
Financing receivable modifications performing recorded investment	52,000		58,000
Troubled debt restructurings balance	$ 2,351,000		$ 2,389,000
Residential Properties \| Real Estate Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maximum loan-to-value ratio (in hundredths)	80.00%
Amortization period of loans	25 years
Balloon period	five years