Document And Entity Information Statistical Measurement [Axis] Note To Financial Statement Details Textual EBP, Basis of Accounting [Policy Text Block] Significant Accounting Policies Note 1 - Plan Description Note 3 - Fair Value Measurement Note 10 - Reconciliation of Financial Statements to Form 5500 Schedule H, Line 4i Note 1 - Plan Description - Vesting Schedule (Details) Note 3 - Fair Value Measurement - Schedule of Assets at Fair Value (Details) Note 10 - Reconciliation of Financial Statements to Form 5500 - Schedule of Reconciliation to Form 5500 (Details) Minimum [Member] Schedule H, Line 4i - Schedule of Assets (Held at End of Year) 2 (Details) Schedule H, Line 4i - Schedule of Assets (Held at End of Year) 2 2 (Details) Statistical Measurement [Domain] Notes To Financial Statements Maximum [Member] Notes To Financial Statements [Abstract] EBP, Investment, Fair Value and NAV [Text Block] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseDecreaseForInvestmentIncomeLossAfterInvestmentExpense Total Investment income EBP, Related Party and Party-in-Interest Transactions [Table] Accounting Policies [Abstract] EBP, Risk and Uncertainty [Table] us-gaap-ebp_EbpPlanName EBP, Plan Name Subsequent Event [Member] Investment, Identifier [Axis] Statement [Table] Fair Value Hierarchy and NAV [Domain] Statement of Financial Position [Abstract] EBP, Participant-Directed and Nonparticipant-Directed Status [Domain] Mutual Fund [Member] EBP, Schedule of Asset Held for Investment [Text Block] EBP, Expense [Policy Text Block] dei_DocumentType Document Type EBP, Investment, Excluding Plan Interest in Master Trust, Cost EBP, Investment, Excluding Plan Interest in Master Trust, Cost Document Information [Line Items] Document Information [Table] EBP, Contribution [Table] Add: Prior year employer contribution receivable Entity Interactive Data Current Statement [Line Items] Fair Value Hierarchy and NAV [Axis] fmnb_EBPAutomaticEnrollmentStartingPercentagePerEmployee EBP, Automatic Enrollment, Starting Percentage Per Employee percentage of employee gross pay the employee is automatically enrolled with for contributions in the employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). fmnb_EBPAutomaticEnrollmentAnnualIncreasePercentage EBP, Automatic Enrollment, Annual Increase Percentage Percentage of employees gross pay for annual increase in contributions in the employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). fmnb_EBPMinimumDaysOfService EBP, Minimum Days of Service (Day) The minimum days of service of participants in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). fmnb_EBPParticipantContributionPretaxMinimumAnnualCompensationToTotalCompensationPercentage EBP, Participant Contribution, Pretax, Minimum Annual Compensation to Total Compensation, Percentage Percentage of minimum pretax annual compensation that participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)), is permitted to contribute to total pretax annual compensation. Diversified Investments [Member] Relating to diversified investments. fmnb_EBPMinimumAgeOfParticipants EBP, Minimum Age of Participants The minimum age of participants in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). EBP 34-0214400 002 [Member] Relating to EBP 34-0214400 002. Greater Than 3 Years But Less Than 4 years of Service [Member] Relating to more than 3 years but less than 4 years of service. Greater Than 4 Years But Less Than 5 Years of Service [Member] Relating to greater than 4 years but less than 5 years of service. Less than 2 Years of Service [Member] Relating to less than 2 years of service. EBP, Payment to Participant [Policy Text Block] Greater than 2 Years But Less than 3 Years of Service [Member] Relating to greater than 2 years but less than 3 years of service. EBP, Schedule of Vesting [Table Text Block] A tabular disclosure of how an employee benefit plan (EBP) vests. Vested percentage Represents the percentage of an employee benefit plan (EBO) vested at a point in time. fmnb_EBPAutomaticEnrollmentMaxPercentagePerEmployee EBP, Automatic Enrollment Max Percentage Per Employee The maximum percentage of automatic enrollment per employee for employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). fmnb_EBPNoteReceivableFromParticipantTerm EBP, Note Receivable from Participant, Term (Year) Period for term on note receivable from participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Add: Current year excess contributions payable Amount of increase in net asset available for benefit from deemed distribution in current period to participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) to reconcile change in net asset available for benefit per financial statement to Form 5500 caption. Form 5500 is form provided by Department of the Treasury, Internal Revenue Service (IRS); Department of Labor (DOL), Employee Benefit Security Administration (EBSA); and Pension Benefit Guaranty Corporation (PBGC). us-gaap-ebp_EmployeeBenefitPlanTaxDeterminationLetterObtained EBP, Tax Determination Letter, Obtained [true false] Schedule of Assets [Text Block] The entire disclosure for schedule of assets. fmnb_EBPPlanTerminationVestingPercentage EBP, Plan Termination, Vesting Percentage Represent the percentage vested in the event of plan termination of employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). Participant status us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitDecrease Total deductions Less: Prior year excess contributions payable Amount of decrease in net asset available for benefit from deemed distribution in prior period to participant in employee stock purchase, saving, or similar plan (employee benefit plan (EBP)) to reconcile change in net asset available for benefit per financial statement to Form 5500 caption. Form 5500 is form provided by Department of the Treasury, Internal Revenue Service (IRS); Department of Labor (DOL), Employee Benefit Security Administration (EBSA); and Pension Benefit Guaranty Corporation (PBGC). fmnb_EBPFullyBenefitResponsiveInvestmentContractAverageYield EBP, Fully Benefit-Responsive Investment Contract, Average Yield Represents the average yield of a fully benefit responsive investment contract held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). fmnb_EBPFullyBenefitResponsiveInvestmentContractActualAverageYield EBP, Fully Benefit-Responsive Investment Contract, Actual Average Yield Represents the actual average yield of a fully benefit responsive contract held by employee stock purchase, saving, or similar plan (employee benefit plan (EBP)). us-gaap-ebp_EmployeeBenefitPlanReconciliationToForm5500NetAssetAvailableForBenefitDeemedDistribution Less: Employer contribution receivable Greater than 5 Years But Less Than 6 Years of Service [Member] Relating to greater than 5 years but less than 6 years of service. EBP, Plan Termination [Text Block] Net assets available for benefits per financial statements NET ASSETS AVAILABLE FOR BENEFITS BEGINNING OF YEAR END OF YEAR Greater Than 6 Years of Service [Member] Relating to greater than 6 years of service. Empower Funds [Member] Relating to Empower Funds. Empower Annuity Insurance Company [Member] Relating to Empower Annuity Insurance Company. JP Morgan [Member] Relating to JP Morgan. EBP, Pooled Separate Account [Member] MFS [Member] Relating to MFS. us-gaap_PolicyTextBlockAbstract Accounting Policies Invesco [Member] Relating to Invesco. Participants' Vanguard [Member] Relating to Vanguard. us-gaap_DisclosureTextBlockAbstract Notes to Financial Statements Retirement Plan Tax Status [Domain] EBP, Contribution [Text Block] PIMCO [Member] Relating to PIMCO. us-gaap_TableTextBlock Notes Tables Qualified Plan [Member] Fidelity Investments [Member] Relating to Fidelity Investments. Document Fiscal Period Focus EBP, Note Receivable from Participant [Policy Text Block] Document Fiscal Year Focus us-gaap-ebp_EmployeeBenefitPlanForfeitedNonvestedAccountReallocatedToParticipant EBP, Forfeited Nonvested Account, Reallocated to Participant Amendment Flag Farmers National Banc Corp [Member] Relating to Farmers National Banc Corp. us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetAssetAvailableForBenefit Net Assets available for benefits per Form 5500 Farmers Trust Company and National Associates [Member] Relating to Farmers Trust Company and National Associates. us-gaap-ebp_EmployeeBenefitPlanAssetHeldForInvestmentPartyInInterest EBP, Asset Held for Investment, Party-in-Interest [true false] dei_DocumentPeriodEndDate Document Period End Date us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForContribution EBP, Change in Net Asset Available for Benefit, Increase from Contribution Legal Entity [Axis] EBP, Schedule of Asset Held for Investment [Table] us-gaap-ebp_EmployeeBenefitPlanParticipantLoanMaturityDate EBP, Asset Held for Investment, Participant Loan, Maturity Date Empower [Member] Relating to Empower. fmnb_EmployeeBenefitPlanUnrealizedAppreciationDepreciation Employee Benefit Plan, Unrealized Appreciation Depreciation Represents the amount of unrealized appreciation depreciation for an employee benefit plan. Notes receivable from participants dei_EntityTaxIdentificationNumber Entity Tax Identification Number Entity [Domain] Other income EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Note Receivable from Participant Entity Central Index Key Net appreciation in fair value of investments dei_EntityRegistrantName Entity Registrant Name Entity Current Reporting Status EBP, Description of Plan [Text Block] EBP, Asset Held for Investment Current Fiscal Year End Date us-gaap-ebp_EmployeeBenefitPlanTaxDeterminationLetterDate EBP, Tax Determination Letter, Date EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncrease Total additions Entity Shell Company Entity Emerging Growth Company Entity Small Business Investments [Domain] EBP, Employer, Common Stock [Member] Benefits paid to participants Less: Current year employer contribution receivable Rollovers EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Excess contributions payable EBP, Excess Contribution Payable Companies' EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash and Noncash FULLY BENEFIT-RESPONSIVE INVESTMENT CONTRACT AT CONTRACT VALUE Valuation, Cost Approach [Member] Valuation Approach and Technique [Domain] us-gaap-ebp_EmployeeBenefitPlanTaxQualificationStatusExtensibleEnumeration EBP, Tax Qualification Status [Extensible Enumeration] Valuation, Market Approach [Member] Statement of Stockholders' Equity [Abstract] EBP, Tax Status [Text Block] us-gaap-ebp_EmployeeBenefitPlanForfeitedNonvestedAccount EBP, Forfeited Nonvested Account us-gaap-ebp_EmployeeBenefitPlanInvestmentValuationTechniqueExtensibleEnumeration EBP, Investment, Valuation Technique [Extensible Enumeration] Current value EBP, Reconciliation of Financial Statement to Form 5500 [Table Text Block] Equity Components [Axis] us-gaap-ebp_EmployeeBenefitPlanInvestmentFairValue Investments at fair value us-gaap-ebp_EmployeeBenefitPlanForm5500CaptionNetIncomeLoss Net increase in net assets available for benefits per Form 5500 Class of Stock [Axis] Equity Component [Domain] us-gaap-ebp_EmployeeBenefitPlanChangeInNetAssetAvailableForBenefitIncreaseForDividendIncomeOnInvestment EBP, Change in Net Asset Available for Benefit, Increase from Dividend Income on Investment Fair Value, Inputs, Level 1 [Member] Add: Excess contributions payable Net increase in net assets available for benefits per financial statements NET INCREASE DEDUCTIONS FROM NET ASSETS ATTRIBUTED TO: ADDITIONS TO NET ASSETS ATTRIBUTED TO: EBP, Reconciliation of Financial Statement to Form 5500 [Text Block] us-gaap-ebp_EmployeeBenefitPlanParticipantLoanInterestRate EBP, Asset Held for Investment, Participant Loan, Interest Rate us-gaap-ebp_EmployeeBenefitPlanForfeitedNonvestedAccountDecreaseForEmployerContribution EBP, Forfeited Nonvested Account, Decrease from Employer Contribution EBP, Participant-Directed [Member] Contributions Subsequent Event Type [Domain] Scenario [Axis] Scenario [Domain] Subsequent Event Type [Axis] Investment income Interest and dividends INVESTMENTS, AT FAIR VALUE Total assets at fair value Investments at fair value EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value us-gaap-ebp_EmployeeBenefitPlanInvestmentExcludingPlanInterestInMasterTrustContractValue EBP, Investment, Excluding Plan Interest in Master Trust, Contract Value Administrative expense EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense fmnb_PolicyTable EBP, Accounting Policy [Table] RECEIVABLES EBP, Description of Plan [Table] us-gaap-ebp_EmployeeBenefitPlanReceivable EBP, Receivable EBP, Reconciliation of Financial Statement to Form 5500 [Table] EBP, Tax Status [Table] us-gaap-ebp_EmployeeBenefitPlanPlanNumber EBP, Plan Number EBP, Risk and Uncertainty [Text Block] us-gaap-ebp_EmployeeBenefitPlanFullyBenefitResponsiveInvestmentContractLimitationOnTransactingAtContractValueNotProbable EBP, Fully Benefit-Responsive Investment Contract, Limitation on Transacting at Contract Value, Not Probable [true false] us-gaap-ebp_EmployeeBenefitPlanFullyBenefitResponsiveInvestmentContractDescription EBP, Fully Benefit-Responsive Investment Contract, Description EBP, Traditional Guaranteed Investment Contract [Member] EBP, Fully Benefit-Responsive Investment Contract [Text Block] Investment Type [Axis] us-gaap-ebp_EmployeeBenefitPlanInvestmentNumberOfSharesPurchased EBP, Investment, Purchased, Number of Shares us-gaap-ebp_EmployeeBenefitPlanEmployerContributionMatchingPercentage EBP, Employer Contribution, Matching Percentage us-gaap-ebp_EmployeeBenefitPlanInvestmentNumberOfShares EBP, Investment, Number of Shares EBP, Use of Estimate [Policy Text Block] us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingAmount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount us-gaap-ebp_EmployeeBenefitPlanParticipantContributionPretaxMaximumAnnualCompensationToTotalCompensationPercentage EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage EBP, Related Party and Party-in-Interest [Domain] EBP, Subsequent Event [Policy Text Block] EBP, Other Related Party and Party-in-Interest [Member] us-gaap-ebp_EmployeeBenefitPlanInvestmentIssuerNameExtensibleEnumeration EBP, Investment, Issuer Name [Extensible Enumeration] us-gaap-ebp_EmployeeBenefitPlanInvestmentNumberOfSharesSold EBP, Investment, Sold, Number of Shares EBP, Summary of Accounting Policy [Text Block] EBP, Investment [Policy Text Block] Contributions receivable companies' EBP, Related Party and Party-in-Interest [Axis] us-gaap-ebp_EmployeeBenefitPlanAsset TOTAL ASSETS us-gaap-ebp_EmployeeBenefitPlanNoteReceivableFromParticipantAccountMaximumBorrowingToTotalAccountPercentage EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage us-gaap-ebp_EmployeeBenefitPlanEmployerContributionParticipantCompensationMatchedPercentage EBP, Employer Contribution, Participant Compensation Matched, Percentage Investment, Issuer Name [Domain] EBP, Related Party and Party-in-Interest Transactions [Text Block] LIABILITIES EBP, Fully Benefit-Responsive Investment Contract [Table] EBP, Investment, Fair Value and NAV [Table Text Block] EBP, Investment, Fair Value and NAV [Table]