Disclosures of Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Available-for-sale: [Abstract]
Total available-for-sale securities	$ 692,274	$ 578,579
Impaired Loans Receivable [Abstract]
Carrying amount of loans with a specific allowance		3,665
Fair value of loans with a specific allowance		3,053
Specific Allowance		612
Fair Value, Measurements, Recurring [Member]
Available-for-sale: [Abstract]
Total available-for-sale securities	692,274	578,579
Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale: [Abstract]
Total available-for-sale securities	364	172
Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Available-for-sale: [Abstract]
Total available-for-sale securities	690,943	575,859
Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Available-for-sale: [Abstract]
Total available-for-sale securities	967	2,548
Fair Value, Measurements, Nonrecurring [Member]
Impaired Loans Receivable [Abstract]
Carrying amount of loans with a specific allowance	19,481
Fair value of loans with a specific allowance	16,437
Specific Allowance	3,044
U.S. Treasury securities and obligations of U.S. government corporations & agencies
Available-for-sale: [Abstract]
Total available-for-sale securities	198,649	113,770
U.S. Treasury securities and obligations of U.S. government corporations & agencies \| Fair Value, Measurements, Recurring [Member]
Available-for-sale: [Abstract]
Total available-for-sale securities	198,649	113,770
U.S. Treasury securities and obligations of U.S. government corporations & agencies \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale: [Abstract]
Total available-for-sale securities	0	0
U.S. Treasury securities and obligations of U.S. government corporations & agencies \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Available-for-sale: [Abstract]
Total available-for-sale securities	198,649	113,770
U.S. Treasury securities and obligations of U.S. government corporations & agencies \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Available-for-sale: [Abstract]
Total available-for-sale securities	0	0
Obligations of states and political subdivisions
Available-for-sale: [Abstract]
Total available-for-sale securities	192,579	166,266
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Transfers into Level 3	967	0
Transfers out of Level 3	0	0
Total gains or losses [Abstract]
Included in net income	0	0
Included in other comprehensive income (loss)	0	0
Purchases, issuances, sales and settlements [Abstract]
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Ending balance	967	0
Total gains or losses for the period included in net income attributable to the change in unrealized gains or losses related to assets and liabilities still held at the reporting date	0	0
Obligations of states and political subdivisions \| Fair Value, Measurements, Recurring [Member]
Available-for-sale: [Abstract]
Total available-for-sale securities	192,579	166,266
Obligations of states and political subdivisions \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale: [Abstract]
Total available-for-sale securities	0	0
Obligations of states and political subdivisions \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Available-for-sale: [Abstract]
Total available-for-sale securities	191,612	166,266
Obligations of states and political subdivisions \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Available-for-sale: [Abstract]
Total available-for-sale securities	967	0
Residential Mortgage Backed Securities [Member]
Available-for-sale: [Abstract]
Total available-for-sale securities	298,672	293,811
Residential Mortgage Backed Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Available-for-sale: [Abstract]
Total available-for-sale securities	298,672	293,811
Residential Mortgage Backed Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale: [Abstract]
Total available-for-sale securities	0	0
Residential Mortgage Backed Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Available-for-sale: [Abstract]
Total available-for-sale securities	298,672	293,811
Residential Mortgage Backed Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Available-for-sale: [Abstract]
Total available-for-sale securities	0	0
Trust preferred securities
Available-for-sale: [Abstract]
Total available-for-sale securities		2,548
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2,548	1,652
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total gains or losses [Abstract]
Included in net income	0	0
Included in other comprehensive income (loss)	18	1,053
Purchases, issuances, sales and settlements [Abstract]
Purchases	0	0
Issuances	0	0
Sales	(2,522)	0
Settlements	(44)	(157)
Ending balance	0	2,548
Total gains or losses for the period included in net income attributable to the change in unrealized gains or losses related to assets and liabilities still held at the reporting date	0	0
Trust preferred securities \| Fair Value, Measurements, Recurring [Member]
Available-for-sale: [Abstract]
Total available-for-sale securities		2,548
Trust preferred securities \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale: [Abstract]
Total available-for-sale securities		0
Trust preferred securities \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Available-for-sale: [Abstract]
Total available-for-sale securities		0
Trust preferred securities \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Available-for-sale: [Abstract]
Total available-for-sale securities		2,548
Other securities
Available-for-sale: [Abstract]
Total available-for-sale securities	2,374	2,184
Other securities \| Fair Value, Measurements, Recurring [Member]
Available-for-sale: [Abstract]
Total available-for-sale securities	2,374	2,184
Other securities \| Fair Value, Measurements, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Available-for-sale: [Abstract]
Total available-for-sale securities	364	172
Other securities \| Fair Value, Measurements, Recurring [Member] \| Significant Other Observable Inputs (Level 2)
Available-for-sale: [Abstract]
Total available-for-sale securities	2,010	2,012
Other securities \| Fair Value, Measurements, Recurring [Member] \| Significant Unobservable Inputs (Level 3)
Available-for-sale: [Abstract]
Total available-for-sale securities	$ 0	$ 0

Amount of allowance for credit losses related to recorded investment measured at fair value on a nonrecurring basis.

No definition available.

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

- Definition

Amount of investment in impaired financing receivables, for which there is a related allowance for credit losses, measured at fair value on a nonrecurring basis.

+ References

No definition available.

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables, for which there is a related allowance for credit losses, measured at fair value on a nonrecurring basis.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26626-111562

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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

- Definition

Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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

- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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

- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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

- Definition

Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized loss realized upon the write-down of available-for-sale securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e689-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 17A
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL34724391-108580

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=114867106&loc=d3e637-108580

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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