Collateral financing receivable related allowance.

No definition available.

Name:	fmnb_CollateralFinancingReceivableRelatedAllowance
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Collateral financing receivable with related allowance unpaid principal balance.

+ References

No definition available.

+ Details

Name:	fmnb_CollateralFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value assets liabilities level 1 and level 2 transfers amount.

+ References

No definition available.

+ Details

Name:	fmnb_FairValueAssetsAndLiabilitiesLevel1AndLevel2TransfersAmount
Namespace Prefix:	fmnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Maximum maturity period of short term borrowings.

+ References

No definition available.

+ Details

Name:	fmnb_MaximumMaturityPeriodOfShortTermBorrowings
Namespace Prefix:	fmnb_
Data Type:	xbrli:durationItemType
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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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 https://asc.fasb.org/extlink&oid=123577603&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 https://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

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_IncomeApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Fair Value (Details Textual) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
Fair Value (Details Textual) - USD ($) $ in Thousands	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value assets liabilities transfers amount between level 1 and level 2	$ 0	$ 0	$ 0	$ 0
Troubled debt restructurings	3,800		$ 3,800		$ 3,900
Maximum maturity period of short term borrowings			90 days
Measured Using Present Value of Cash Flows
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Troubled debt restructurings	846		$ 846		792
Collateral Dependent Loans
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Collateral dependent loans, unpaid principal balance with allowance recorded	1,200		1,200		3,200
Collateral dependent loans, allowance	$ 214		$ 214		$ 1,500