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Share-Based Payment Arrangement [Text Block] Changes in assets and liabilities: Preferred stock, shares issued (in shares) Preferred stock, shares outstanding (in shares) Change in fair value of cash flow hedges Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member] Preferred stock, shares authorized (in shares) Portion at Fair Value Measurement [Member] Reported Value Measurement [Member] Change is fair value of cash flow hedges, tax Preferred stock, par value (in dollars per share) Change in fair value of cash flow hedge us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect Increase in cash and cash equivalents Equipment Equipment Cash and interest-bearing deposits in banks us-gaap_CashAndCashEquivalentsFairValueDisclosure Estimate of Fair Value Measurement [Member] New Accounting Pronouncements, Policy [Policy Text Block] us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations Beginning Ending US Government Agencies Debt Securities [Member] Segment Reporting, Entity-Wide Information Not Provided as Part of Reportable Segment, Customer, Extent of Reliance [Table] Statement [Table] Time deposits us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss Total, unrealized (loss), available for sale securities Loans, net us-gaap_LoansReceivableFairValueDisclosure Total, fair value, available for sale securities Derivative liabilities us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions Debt Securities, Available-for-Sale, Unrealized Loss Position, Number of Positions Financing Receivable, Allowance for Credit Loss [Table Text Block] us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber Unvested, beginning of year (in shares) Unvested, end of period (in shares) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue Unvested, beginning of year, weighted average grant date fair value (in dollars per share) Unvested, end of period, weighted average grant date fair value (in dollars per share) Granted, shares (in shares) Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) Financing Receivable, Past Due [Table Text Block] Granted, weighted average grant date fair value (in dollars per share) us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod Forfeited, shares (in shares) Financing Receivable Credit Quality Indicators [Table Text Block] Other Noninterest Income [Member] Relating to other noninterest income. Product and Service [Domain] Financing Receivable, Modified [Table Text Block] Product and Service [Axis] Schedule of Net Effect of Amortization and Accretion Adjustment Associated With Business Combination [Table Text Block] Tabular disclosure of net effect of amortization and accretion adjustment associated with business combination. Securities held to maturity fxnc_DebtSecuritiesHeldtomaturityFairValue Fair value of investment in debt security measured at amortized cost (held-to-maturity). 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Gross unrealized gains, available for sale and held to maturity securities us-gaap_DebtSecuritiesAvailableForSaleAndHeldToMaturityCumulativeUnrecognizedLoss Gross unrealized losses Proceeds from sale of premises and equipment Federal Home Loan Bank stock Issuance of common stock, stock incentive plan fxnc_SharesIssuedValueSharebasedIncentivePlanBeforeForfeiture The value of the stock issued for the share-based incentive plan before forfeiture. Statement of Stockholders' Equity [Abstract] Proceeds from sale of other real estate owned Statement of Comprehensive Income [Abstract] Loan Restructuring Modification [Axis] Loan Restructuring Modification [Domain] Proceeds from sale of mortgage loans held for sale Contractual Interest Rate Reduction [Member] Statement of Cash Flows [Abstract] Financing Receivable Portfolio Segment [Axis] Other comprehensive income Total other comprehensive (loss) Shareholders’ Equity Equity Components [Axis] Financing Receivable Portfolio Segment [Domain] Equity Component [Domain] Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 [Member] AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-Sale, Parent [Member] AOCI Attributable to Parent [Member] Fair Value, Inputs, Level 1 [Member] Additional Paid-in Capital [Member] Corporate Debt Securities [Member] Retained Earnings [Member] Real Estate [Member] Premises and equipment, net Subordinated debt, net of issuance cost Financial Asset, Aging [Axis] Loans Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss Financing Receivable, Nonaccrual [Table Text Block] Financial Asset, Aging [Domain] Interest Rate Swap Two [Member] Represents the second interest rate swap. Provision for credit losses on loans Provision for credit losses Provision for credit losses Interest Rate Swap One [Member] Represents the first interest rate swap. Amortized cost, held to maturity securities Amortized cost, held to maturity Financial Asset, 30 to 59 Days Past Due [Member] us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue Stock-based compensation Held to maturity, allowance for credit loss Securities available for sale, HTM Balance, HTM Balance, HTM Allowance for Credit Loss Financing Receivable, Allowance for Credit Loss, Excluding Accrued Interest Allowance for credit losses Balance Balance Financial Asset, 60 to 89 Days Past Due [Member] Share-Based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block] Financial Asset, Equal to or Greater than 90 Days Past Due [Member] us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostAllowanceForCreditLossExcludingAccruedInterest Securities available for sale, AFS Loans, four years before Loans, prior Loans, two years before Loans, three years before Fair Value Disclosures [Text Block] Loans, current year Fair Value, by Balance Sheet Grouping [Table Text Block] Loans, one year before Supplemental Disclosures of Cash Flow Information Loans, revolving us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments Accretion of certain acquisition-related loan (discounts) premiums, net Loans held for sale us-gaap_LoansHeldForSaleFairValueDisclosure Allowance for Credit Losses [Text Block] Marketing Collateral Held [Domain] Collateral Held [Axis] Accrued interest receivable us-gaap_GainLossOnSaleOfPropertyPlantEquipment Net (gains) losses on disposal of premises and equipment Accumulated other comprehensive loss, net Business Combination, Pro Forma Information [Table] us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest Net impact to income before taxes Data processing expense Data processing expense Surplus Federal Reserve Bank stock Use of Estimates, Policy [Policy Text Block] us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1 Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Period (Year) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] us-gaap_PaymentsToAcquireProductiveAssets Purchase of premises and equipment Collateral Pledged [Member] Issuance of common stock, dividend reinvestment plan Other borrowings us-gaap_DebtInstrumentFairValue us-gaap_StockRepurchasedAndRetiredDuringPeriodValue Repurchase of common stock us-gaap_NoninterestExpense Total noninterest expense Modification amount Noninterest Expense Financial Asset, Not Past Due [Member] Debt Securities, Held-to-Maturity, Credit Quality Indicator [Table Text Block] Fair Value, Recurring and Nonrecurring [Table] us-gaap_OffBalanceSheetCreditLossLiability Off-Balance-Sheet, Credit Loss, Liability Restricted Stock Units (RSUs) [Member] Issuance of common stock, dividend reinvestment plan, shares (in shares) Repurchase of common stock shares (in shares) fxnc_FinancingReceivablesAllowanceForCreditLossesCollectivelyEvaluatedForImpairment Collectively evaluated The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment. Cash Flows from Operating Activities Cash Flows from Investing Activities us-gaap_NetCashProvidedByUsedInOperatingActivities Net cash provided by operating activities Cash Flows from Financing Activities us-gaap_NetCashProvidedByUsedInInvestingActivities Net cash (used in) provided by investing activities Intangible Asset, Finite-Lived, and Capitalized Cost, Software to be Sold, Leased, or Marketed, Estimated Amortization Expense [Table Text Block] us-gaap_NetCashProvidedByUsedInFinancingActivities Net cash provided by financing activities us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt Purchases of securities available for sale Total assets us-gaap_AssetsFairValueDisclosure Debt Securities, Held-to-Maturity, Allowance for Credit Loss [Table Text Block] Operating Segments [Member] Accounting Policies [Abstract] Segments [Domain] Credit Loss Status [Axis] Credit Loss Status [Domain] US Treasury Securities [Member] US States and Political Subdivisions Debt Securities [Member] Segments [Axis] Loans, net of allowance for credit losses, 2026, $14,698; 2025, $14,719 Loans, net us-gaap_PaymentsForProceedsFromLoansAndLeases Net (increase) decrease in loans Income Statement [Abstract] us-gaap_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversal Off-Balance-Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal) Amortized cost, available for sale securities Amortized cost, available for sale securities Fair value, available for sale securities Securities available for sale, at fair value Fair value, available for sale securities us-gaap_NumberOfReportableSegments Number of Reportable Segments fxnc_PolicyTable Securities held to maturity, at amortized cost (net of allowance for credit losses, 2026, $68; 2025, $83) Deferred income tax (benefit) Net (decrease) increase in time deposits us-gaap_AmortizationOfDeferredCharges Core deposit intangible (2) Net income Net income Net income Financial Asset, Past Due [Member] Amortization of debt issuance costs Core deposit intangible amortization expense Intangible Asset, Finite-Lived, Amortization Expense Amortization fxnc_DebtSecuritiesAvailableForSaleAndHeldtomaturityAmortizedCostExcludingAccruedInterestBeforeAllowanceForCreditLoss Amortized cost, , available for sale and held to maturity securities Amortized cost, excluding accrued interest, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). us-gaap_InterestIncomeExpenseNet Net interest income fxnc_DebtSecuritiesAvailableForSaleAndHeldtomaturityAllowanceForCreditLossExcludingAccruedInterest Securities available for sale, available for sale and HTM Amount of allowance for credit loss on investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Accumulated Other Comprehensive Income (Loss) [Table] fxnc_DebtSecuritiesAvailableForSaleAndHeldtomaturityFairValueExcludingAccruedInterest Fair value, available for sale and held to maturity securities Fair value of investment in debt security measured at amortized cost (held-to-maturity) and investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), excluding accrued interest. Amortization of unrealized losses on securities transferred from available for sale to held to maturity Amount, before tax and adjustment, of unrealized gain (loss) on investment from transferred from available-for-sale to held-to-sale investments. Supplemental Disclosures of Noncash Investing and Financing Activities Interest on junior subordinated debt Occupancy Occupancy us-gaap_OtherNoninterestExpense Other operating expense FDIC assessment us-gaap_FinancingReceivablePurchasedWithCreditDeteriorationDiscountPremium Financing Receivable, Purchased with Credit Deterioration, Discount (Premium) us-gaap_LifeInsuranceCorporateOrBankOwnedChangeInValue Increase in cash value of bank owned life insurance Income from bank owned life insurance us-gaap_NoninterestIncome Total noninterest income Total noninterest income Noninterest Income Loans, Notes, Trade and Other Receivables, Excluding Allowance for Credit Losses [Text Block] Supplies Measurement Basis [Axis] us-gaap_GainsLossesOnSalesOfOtherRealEstate Net (gain) on sale of other real estate owned Miscellaneous losses Represents operating gains (losses) classified as miscellaneous. Other operating income Fair Value Hierarchy and NAV [Domain] Measurement Frequency [Domain] Internet banking expense The amount of noninterest expense related to internet banking which are paid by the Entity and then paid by the customer. Vesting [Axis] Statement of Financial Position [Abstract] Vesting [Domain] Accrued interest payable and other liabilities The amount of accrued but unpaid interest on deposit liabilities, and liabilities classified as other. Provision for credit losses, HTM ATM and check card expense The amount of noninterest expense related to ATM and check card fees and payments which are paid by the Entity and then paid by the customer. Bank franchise tax A tax paid by the bank in lieu of a business license and business tangible property taxes. Recoveries, HTM Recoveries Other assets Current period gross write-offs, total Charge-offs us-gaap_DebtSecuritiesHeldToMaturityExcludingAccruedInterestAllowanceForCreditLossWriteoff Charge-offs of securities, HTM Loans (1) The aggregate interest and fee income (expense) generated by: (1) loans the Entity has positive intent and ability to hold for the foreseeable future, or until maturity or payoff, including commercial and consumer loans, whether domestic or foreign, which may consist of: (a) industrial and agricultural; (b) real estate; and (c) real estate construction loans; (d) trade financing; (e) lease financing; (f) home equity lines-of-credit; (g) automobile and other vehicle loans; and (h) credit card and other revolving-type loans and (2) loans and leases held-for-sale which may include mortgage loans, direct financing, and sales-type leases. Legal and professional fees Legal and professional fees Fair Value, Recurring [Member] Accretion of premium on time deposits The amount of noncash expense included in interest expense for accretion or amortization of debt discounts and premiums associated with related debt instruments. us-gaap_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteoffAfterRecovery Provision for (recovery of) credit losses on loans Other operating expense fxnc_WeightedAverageRepricingTerm Weighted Average Re-pricing Term (Year) The weighted average period of time between re-pricing of the portfolio. us-gaap_PaymentsForRepurchaseOfCommonStock Repurchase of common stock fxnc_AvailableforsaleSecuritiesNumberOfSecurities Available-for-Sale Securities, Number of Securities The number of investment positions in available-for-sale investments. us-gaap_IncreaseDecreaseInOtherOperatingAssets Decrease in other assets Document Type Due after five years through ten years, Amortized cost, held to maturity securities Due after ten years, Amortized cost, held to maturity securities Document Information [Line Items] Document Quarterly Report Schedule of Available-for-sale and Held-to-maturity Reconciliation [Table Text Block] The tabular disclosure of amortized costs, unrealized gains (losses), and fair values of available-for-sale and held-to-maturity securities. Due within one year, Amortized cost, held to maturity securities US Agency and Mortgage-backed Securities [Member] Information pertaining to securities backed by US agencies and mortgages. Due after one year through five years, Amortized cost, held to maturity securities Document Information [Table] us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares) Entity Interactive Data Current Bank owned life insurance Bank owned life insurance Document Transition Report us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding Weighted average shares outstanding – diluted (in shares) Community Bankers’ Bank stock Represents an equity interest in community banks. Earnings per common share Three Year Vesting [Member] Represents share-based compensation awards of which they vest over a three year period. Weighted average shares outstanding – basic (in shares) Potentially dilutive common shares – restricted stock units (in shares) Derivative Instrument [Axis] Basic (in dollars per share) us-gaap_StockIssued1 Issuance of common stock, dividend reinvestment plan Accruing loans past due 90 days or more amounted Financial Asset Acquired and No Credit Deterioration [Member] Nonaccrual, No Allowance Earnings Per Share, Diluted, by Common Class, Including Two-Class Method [Table] Non-accrual loans Savings and interest-bearing demand deposits us-gaap_Deposits Total deposits Core deposit intangibles, net Accretion of discounts and amortization of premiums on securities, net Noninterest-bearing demand deposits Issuance of common stock, stock incentive plan, shares (in shares) us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensationGross us-gaap_FiniteLivedIntangibleAssetsNet Total Beginning of period, December 31 Total core deposit intangible Depreciation and amortization of premises and equipment Document Fiscal Period Focus Issuance of common stock, stock incentive plan us-gaap_StockIssuedDuringPeriodValueShareBasedCompensationGross Document Fiscal Year Focus Amendment Flag Interest rate swap contracts Amount of obligation to return cash or securities collateral under master netting arrangements that have not been offset against derivative assets. Common Stock [Member] us-gaap_FinancingReceivableUnamortizedLoanCommitmentOriginationFeeAndPremiumDiscount Financing Receivable, Unamortized Loan Cost (Fee) and Purchase Premium (Discount) Goodwill Document Period End Date Restricted securities Restricted securities, at cost Total restricted securities Interest and Dividend Income Dividends Legal Entity [Axis] us-gaap_InterestAndDividendIncomeOperating Total interest and dividend income Financing Receivable, Allowance for Credit Loss [Table] Business Combination [Domain] us-gaap_OtherComprehensiveIncomeLossTransfersFromHeldToMaturityToAvailableForSaleSecuritiesNetOfTax Amortization of unrealized holding losses on available-for-sale securities transferred to held to maturity Individually evaluated Entity File Number us-gaap_DerivativeFixedInterestRate Derivative, Fixed Interest Rate Collectively evaluated Entity Tax Identification Number Business Combination [Axis] us-gaap_FinancingReceivableAllowanceForCreditLossesCollectivelyEvaluatedForImpairment Collectively evaluated Disaggregation of Revenue [Table Text Block] us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss Less than 12 months, unrealized (loss), available for sale securities Disaggregation of Revenue [Table] us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss 12 months or more, unrealized (loss), available for sale securities Entity [Domain] Entity Central Index Key TDR Loans [Member] Information related to TDR loans. Entity Registrant Name us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount Equity Securities without Readily Determinable Fair Value, Amount Doubtful [Member] Entity Current Reporting Status Business Combination [Table] Special Mention [Member] Substandard [Member] Entity Incorporation, State or Country Code Pass [Member] Current Fiscal Year End Date Entity Filer Category Entity Shell Company Entity Emerging Growth Company Notional amount Derivative, Notional Amount Entity Small Business us-gaap_DerivativeNumberOfInstrumentsHeld Derivative, Number of Instruments Held, Total Hedging Designation [Domain] Derivative Assets Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Interest Expense fxnc_PaymentsForRestrictedSecurities Purchases of restricted securities The amount of cash outflow purchase of restricted securities. Interest-bearing deposits in banks Derivative Instruments and Hedging Activities Disclosure [Text Block] Interest on deposits Time deposits (4) Interest and dividends on securities: Entity Address, State or Province us-gaap_PaymentsForOriginationAndPurchasesOfLoansHeldForSale Origination of mortgage loans held for sale Entity Address, City or Town Marketable Securities [Table] Retained earnings us-gaap_InterestExpenseOperating Total interest expense Interest Expense Entity Address, Address Line One City Area Code Due after one year through five years, amortized cost, available for sale securities Due after five years through ten years, amortized cost, available for sale securities Local Phone Number Due within one year, amortized cost, available for sale securities Revenue from Contract with Customer [Text Block] Entity Address, Postal Zip Code Other borrowings Derivative [Table] (Decrease) in accrued interest payable and other liabilities Title of 12(b) Security Security Exchange Name Due after five years through ten years, fair value, available for sale securities Trading Symbol Due after ten years, fair value, available for sale securities Due within one year, fair value, available for sale securities Due after one year through five years, fair value, available for sale securities Due after ten years, amortized cost, available for sale securities Class of Stock [Axis] Class of Financing Receivable [Axis] Class of Financing Receivable [Domain] Secured by Multi-family Residential Properties [Member] Information about loans secured by multi-family residential properties. Construction and Land Development [Member] Information about construction and land development loans. Entity Common Stock, Shares Outstanding Real Estate 1 [Member] Represents information pertaining to real estate. Other comprehensive (loss) income, net of tax, Earnings Per Share [Text Block] Consumer and Other Loans [Member] Represents information pertaining to consumer and other loans. Commercial and Industrial Loans [Member] Represents information pertaining to commercial and industrial loans. Hedging Designation [Axis] Other Real Estate Loans [Member] Information about real estate loans classified as other. OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax Unrealized (losses) gains on securities available for sale Unrealized holding (losses) gains on available for sale securities, net of tax of ($366) and $428 for the three months ended March 31, 2026 and 2025, respectively Unrealized holding gains Internal Credit Assessment [Axis] Interest on other borrowings Merger expense Fair value, held to maturity securities Fair value, held to maturity securities us-gaap_InterestIncomeExpenseAfterProvisionForLoanLoss Net interest income after provision for credit losses Interest on subordinated debt Subordinated Debt (3) Investments Classified by Contractual Maturity Date [Table Text Block] Interest on deposits in banks Interest on federal funds sold Internal Credit Assessment [Domain] Due after five years through ten years, fair value, held to maturity securities Common stock, par value (in dollars per share) Due after ten years, fair value, held to maturity securities Due within one year, fair value, held to maturity securities Taxable interest Due after one year through five years, fair value, held to maturity securities us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1 Individually evaluated Basis of Accounting, Policy [Policy Text Block] Tax-exempt interest Derivative Contract [Domain] OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, Tax Reclassification adjustment for losses included in net income, tax Cash and due from banks us-gaap_Assets Assets Total assets us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax Change in fair value of cash flow hedges, net of tax of ($1) and ($45) for the three months ended March 31, 2026 and 2025, respectively Designated as Hedging Instrument [Member] us-gaap_CashAndCashEquivalentsAtCarryingValue Cash and cash equivalents us-gaap_InterestIncomeOperating Interest Income Modification percent Diluted (in dollars per share) Derivatives - cash flow hedges us-gaap_CashFlowHedgeDerivativeInstrumentAssetsAtFairValue us-gaap_FinancingReceivableExcludingAccruedInterestModifiedSubsequentDefault Financing Receivable, Excluding Accrued Interest, Modified, Subsequent Default Accrued interest payable fxnc_AccruedInterestPayableFairValueDisclosure The fair value portion of accrued interest payable. Junior subordinated debt fxnc_JuniorSubordinatedDebtFairValueDisclosure Fair value of junior subordinated debt which has a lower priority of repayment than subordinated debt in the event the company falls into liquidation or bankruptcy. External Credit Rating, Moody's [Domain] Net increase in demand deposits and savings accounts The net cash inflow or outflow in the total of all demand deposits, including checking and negotiable order of withdrawal (NOW) accounts, and in savings accounts, classified as cash flows from financing activities. Subordinated debt fxnc_SubordinatedDebtFairValueDisclosure Fair value of subordinated debt obligations. First National Corporation 2014 Stock Incentive Plan [Member] Information pertaining to the First National Corporation 2014 Stock Incentive Plan. Schedule of Derivative Instruments [Table Text Block] Accounts and Financing Receivables [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Core deposit intangible acquired Accrued interest receivable us-gaap_InterestReceivable Interest Interest Rate Swap [Member] Intangible Asset, Finite-Lived [Table Text Block] ATM and Check Card Fees [Member] Represents ATM and check card fees. Service Charges on Deposit Accounts [Member] Represents service charges on deposit accounts. fxnc_NoninterestIncomeExcludingRevenueFromContactWithCustomer Noninterest income (out-of-scope of Topic 606) The total amount of noninterest income which falls out-of-scope of Topic 606. Current period gross write-offs, prior Current period gross write-offs, revolving Fees for Other Customer Services [Member] Represents fees for customer services classified as other. Current period gross write-offs, three years before Wealth Management Fees [Member] Represents wealth management fees. Current period gross write-offs, four years before Current period gross write-offs. one year before Current period gross write-offs, two years before Current period gross write-offs, current year Business Combination [Text Block] us-gaap_DebtSecuritiesHeldToMaturityAccruedInterestAfterAllowanceForCreditLoss Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss us-gaap_StockholdersEquity Total shareholders’ equity Balance Balance Demand Deposit Overdrafts [Member] Represents demand deposit overdrafts. Forfeited, weighted average grant date fair value (in dollars per share) Touchstone Bankshares, Inc [Member] Represents Touchstone Bankshares, Inc. Municipal Bonds [Member] Moody's, Aaa Rating [Member] us-gaap_PaymentsOfDividendsCommonStock Cash dividends paid on common stock, net of reinvestment us-gaap_OciDebtSecuritiesAvailableForSaleTransferToHeldToMaturityAdjustmentFromAociForAmortizationOfGainLossAfterTax Amortization of unrealized holding losses on available-for-sale securities transferred to held to maturity, net of tax of $49 and $50 for the three months ended March 31, 2026 and 2025, respectively Wealth Management [Member] Represents the wealth management segment. Amortization of unrealized holding losses on available-for-sale securities transferred to held to maturity, tax Moody's, A1/A2/A3 Rating [Member] Represents Moody's, A1/A2/A3 Rating. Community Banking [Member] Represents the community banking segment. us-gaap_DebtSecuritiesAvailableForSaleAccruedInterestAfterAllowanceForCreditLoss Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss fxnc_InvestmentManagementExpense Investment management The amount of noninterest expense related to investment management costs. Not Rated - Agency [Member] Represents not rated-agency. Credit and Debit Card [Member] ecd_Rule10b51ArrTrmntdFlag Rule 10b5-1 Arrangement Terminated [Flag] ecd_Rule10b51ArrAdoptedFlag Rule 10b5-1 Arrangement Adopted [Flag] Noninterest income Noninterest income Asset Management [Member] Commitments and contingencies Marketable Securities [Table Text Block] Moody's, Aa1/Aa2/Aa3 Rating [Member] Aa1/Aa2/Aa3 credit rating as defined by the external credit rating agency, Moody's.