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Other Real Estate Owned (OREO) - Summary of Changes in the Balance for OREO (Detail) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Real Estate [Abstract]      
Balance at the beginning of year, gross $ 2,263us-gaap_OtherRealEstate $ 4,695us-gaap_OtherRealEstate $ 4,695us-gaap_OtherRealEstate
Transfers in 228us-gaap_OtherRealEstateAdditions   139us-gaap_OtherRealEstateAdditions
Charge-offs (250)us-gaap_RealEstateOwnedValuationAllowanceAmountsApplied (2)us-gaap_RealEstateOwnedValuationAllowanceAmountsApplied (1,302)us-gaap_RealEstateOwnedValuationAllowanceAmountsApplied
Sales proceeds (224)us-gaap_OtherRealEstateDisposals   (1,502)us-gaap_OtherRealEstateDisposals
Gain on disposition 57fxnc_OtherRealEstateGrossGains   307fxnc_OtherRealEstateGrossGains
Deferred gain recognized     (73)us-gaap_RecognitionOfDeferredRevenue
Depreciation     (1)fxnc_DepreciationOnOtherRealEstate
Balance at the end of period, gross 2,074us-gaap_OtherRealEstate   2,263us-gaap_OtherRealEstate
Less: valuation allowance (125)us-gaap_OtherRealEstateValuationAdjustments   (375)us-gaap_OtherRealEstateValuationAdjustments
Balance at the end of period, net $ 1,949us-gaap_OtherRealEstateAndForeclosedAssets   $ 1,888us-gaap_OtherRealEstateAndForeclosedAssets