The useful life of the loan receivable.

No definition available.

Name:	fmbh_AmortizationPeriodOfLoans
Namespace Prefix:	fmbh_
Data Type:	us-types:durationStringItemType
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 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

Specifies the time period covered by the loan receivable.

+ References

No definition available.

+ Details

Name:	fmbh_LoansReceivableTimePeriod
Namespace Prefix:	fmbh_
Data Type:	us-types:durationStringItemType
Balance Type:	na
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 held-for-sale and loans and leases covered under loss sharing agreements.

+ References

No definition available.

+ Details

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

- 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

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=109223946&loc=d3e3602-108585

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for 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 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

Reference 4: 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=84173941&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- 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

+ Details

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

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ 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=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

The face amount of loans that will be sold to other entities.

+ References

+ Details

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

- Definition

Amount, after allowance for bad debt, of loan receivables held-for-sale that are not part of a disposal group. Excludes loans and leases covered under loss sharing agreements and loans classified as debt securities.

+ References

+ Details

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

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

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

- Definition

Amount of deferred fees paid by borrowers and unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of loans that are being amortized into income as an adjustment to yield. Excludes amounts for 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 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6378536&loc=d3e10092-111533

+ Details

Name:	us-gaap_UnamortizedLoanCommitmentAndOriginationFeesAndUnamortizedDiscountsOrPremiums
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_FarmLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=naics_ZZ713290
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialRealEstateMember
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:

Loans and Allowance for Loan Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 2,699,164	$ 2,658,136
Loans Receivable Held-for-sale, Amount	1,820	1,508
loans and leases receivable gross excluding loans held for sale	2,697,344	2,656,628
Net deferred loan fees, premiums and discounts	(3,817)	(13,617)
Allowance for loan losses	26,911	26,189	$ 19,977	$ 16,753
Net loans	2,666,616	2,616,822
Loans held for sale	1,820	1,508
Increase (Decrease) in Accounts and Notes Receivable	49,800
Loans Receivable, Net	2,695,347	2,644,519
Construction and land development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	94,462	51,013
Loans Receivable, Net	94,142	50,619
Farm loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	240,481	232,409
Loans Receivable, Net	240,241	231,700
1-4 Family residential properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	336,553	374,751
Loans Receivable, Net	336,427	373,518
Multifamily residential properties
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	155,132	186,393
Loans Receivable, Net	153,948	184,051
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	997,175	911,656
Loans Receivable, Net	$ 995,702	906,850
Debt coverage ratio	1.20x
Amortization period of loans	twenty years
Commercial real estate \| Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan To Value Ratio	65.00%
Commercial real estate \| Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan To Value Ratio	80.00%
Loans secured by real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 1,823,803	1,756,222
Loans Receivable, Net	1,820,460	1,746,738
Agricultural loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	136,023	136,125
Loans Receivable, Net	136,124	135,877
Commercial and industrial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	528,987	559,120
Loans Receivable, Net	$ 528,973	557,011
Loan To Value Ratio	80.00%
Amortization period of loans	seven years
Loans Receivable, Time Period	one year
Consumer loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 83,544	92,744
Allowance for loan losses	1,428	932	803	693
Loans Receivable, Net	83,183	91,516
All other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	126,807	113,925
Loans Receivable, Net	126,607	113,377
Agricultural and Farm Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	376,365	367,577
Increase (Decrease) in Accounts and Notes Receivable	$ 8,788
Loan To Value Ratio	65.00%
Amortization period of loans	twenty five years
Loans Receivable, Time Period	one year
Other Grain Farming [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 301,500	276,142
Increase (Decrease) in Accounts and Notes Receivable	25,400
Motels and Hotels Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	120,735	129,216
Non-residential Buildings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	300,611	250,495
Residential Buildings [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	284,378	289,169
Other Gambling Industries [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	90,429	105,259
Commercial/Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	21,618	21,556	16,546	12,901
Agriculture/Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	2,479	2,197	1,742	2,249
Residential Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 1,386	1,504	886	874
Loan To Value Ratio	80.00%
Amortization period of loans	twenty five years
Balloon period	five years
Unallocated [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 0	$ 0	$ 0	$ 36