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Nature of Banking Activities and Significant Accounting Policies - Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Entity
Dec. 31, 2013
Dec. 31, 2012
Nature Of Banking Activities And Significant Accounting Policies [Line Items]      
Number of accounts included by parent company 6fxnc_NumberOfCompaniesForWhichConsolidatedFinancialStatementsPresented    
Recognition of other than temporary impairment on basis of credit loss description If there is no credit loss, there is no other-than-temporary impairment. If there is a credit loss, other-than-temporary impairment exists, and the credit loss must be recognized in net income and the remaining portion of impairment must be recognized in other comprehensive income.    
Gain (loss) on rate lock commitments $ 0fxnc_GainLossOnRateLockCommitments    
Accrual status of loan 90 days    
Troubled debt restructurings (TDRs), amount 1,900,000us-gaap_FinancingReceivableModificationsRecordedInvestment 1,900,000us-gaap_FinancingReceivableModificationsRecordedInvestment  
Largest amount of tax benefit realized by taxing authority 50.00%fxnc_LargestAmountOfTaxBenefitRealizedByTaxingAuthority    
Liability for unrecognized tax benefits 0us-gaap_UnrecognizedTaxBenefits 0us-gaap_UnrecognizedTaxBenefits  
Potential common shares that would have a dilutive effect 0us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount    
Average number of common shares outstanding basic and diluted 4,902,389us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted 4,901,464us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted 3,944,506us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted
Advertising cost 313,000us-gaap_AdvertisingExpense 269,000us-gaap_AdvertisingExpense 364,000us-gaap_AdvertisingExpense
Minimum [Member]      
Nature Of Banking Activities And Significant Accounting Policies [Line Items]      
Period of time between issuance of a loan commitment and closing and sale of the loan 30 days    
Company not considered value of impairment on individual consumer, residential and small commercial loans 250,000fxnc_AmountOfSmallBalanceHomogeneousImpairedLoansNotSeparatelyIdentified
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Management's policy on evaluation of impairment $ 250,000fxnc_AmountOfSubstandardAndDoubtfulLoansEvaluatedAsImpaired
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		    
Minimum [Member] | Software [Member]      
Nature Of Banking Activities And Significant Accounting Policies [Line Items]      
Estimated useful lives of depreciation of premises and equipment 3 years    
Minimum [Member] | Premises and Equipment [Member]      
Nature Of Banking Activities And Significant Accounting Policies [Line Items]      
Estimated useful lives of depreciation of premises and equipment 3 years    
Maximum [Member]      
Nature Of Banking Activities And Significant Accounting Policies [Line Items]      
Period of time between issuance of a loan commitment and closing and sale of the loan 60 days    
Maximum [Member] | Software [Member]      
Nature Of Banking Activities And Significant Accounting Policies [Line Items]      
Estimated useful lives of depreciation of premises and equipment 7 years    
Maximum [Member] | Premises and Equipment [Member]      
Nature Of Banking Activities And Significant Accounting Policies [Line Items]      
Estimated useful lives of depreciation of premises and equipment 40 years