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Fair Value (Details Textual) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2023 | Dec. 31, 2022
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
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 | 6,100 | 1,600
Collateral dependent loans, allowance | $ 1,300 | $ 372

X
- Definition Collateral financing receivable related allowance. + References No definition available. + Details
Name: | fmnb_CollateralFinancingReceivableRelatedAllowance
Namespace Prefix: | fmnb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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

X
- 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

X
- 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

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