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Acquisition - Additional Information (Detail) (Subsequent Event [Member], Bank Of America [Member], USD $)
 0 Months Ended
Apr. 17, 2015
Apr. 17, 2015
Business Acquisition [Line Items]    
Amount for acquisition of deposits $ 2,200,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment $ 2,200,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment
Total consideration 6,700,000us-gaap_BusinessCombinationConsiderationTransferred1  
Estimated deposit premium 4,400,000fxnc_PremiumPaidOnDepositsAcquired 4,400,000fxnc_PremiumPaidOnDepositsAcquired
Amount for acquisition of premises and equipment 188,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFinancialLiabilities 188,000,000us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedFinancialLiabilities
Estimated fair value of acquired premises and equipment 4,600,000us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentPropertyPlantAndEquipment  
Loans acquired $ 0fxnc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLoans $ 0fxnc_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLoans
Virginia [Member]
    
Business Acquisition [Line Items]    
Number of acquired branches 6fxnc_NumberOfBranchesAcquired
/ us-gaap_CounterpartyNameAxis
= fxnc_BankOfAmericaMember
/ us-gaap_StatementGeographicalAxis
= stpr_VA
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember