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Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Detail) (Fair Value, Measurements, Nonrecurring [Member], Significant Unobservable Inputs (Level 3) [Member], USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Impaired Loans Asset, Net [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value assets $ 2,306us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= fxnc_ImpairedLoansAssetNetMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember		 $ 2,976us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= fxnc_ImpairedLoansAssetNetMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember
Fair Value Measurements, Valuation Techniques Property appraisals Property appraisals
Selling cost 10.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
= fxnc_ImpairedLoansAssetNetMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember		 10.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
= fxnc_ImpairedLoansAssetNetMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember
Other Real Estate Owned, Net [Member]
    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Fair value assets $ 1,949us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= fxnc_OtherRealEstateOwnedNetMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember		 $ 1,888us-gaap_AssetsFairValueDisclosure
/ us-gaap_FairValueByAssetClassAxis
= fxnc_OtherRealEstateOwnedNetMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember
Fair Value Measurements, Valuation Techniques Property appraisals Property appraisals
Selling cost 7.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
= fxnc_OtherRealEstateOwnedNetMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember		 7.00%us-gaap_FairValueInputsDiscountRate
/ us-gaap_FairValueByAssetClassAxis
= fxnc_OtherRealEstateOwnedNetMember
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel3Member
/ us-gaap_FairValueByMeasurementFrequencyAxis
= us-gaap_FairValueMeasurementsNonrecurringMember