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Allowance for Loan Losses - Information Regarding Loans Modified under TDR (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Contract	Dec. 31, 2013 Contract	Dec. 31, 2012 Contracts
Financing Receivable, Modifications [Line Items]
Number of Contracts	1us-gaap_FinancingReceivableModificationsNumberOfContracts2	0us-gaap_FinancingReceivableModificationsNumberOfContracts2	2us-gaap_FinancingReceivableModificationsNumberOfContracts2
Pre-modification outstanding recorded investment	$ 283us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2		$ 2,609us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Post-modification outstanding recorded investment	344us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2		2,609us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Real Estate Loans [Member] | Secured by 1-4 Family Residential [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts			1us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_ResidentialOneThroughFourFamilyMember
Pre-modification outstanding recorded investment			183us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_ResidentialOneThroughFourFamilyMember
Post-modification outstanding recorded investment			183us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_ResidentialOneThroughFourFamilyMember
Real Estate Loans [Member] | Other Real Estate Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Contracts	1us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_OtherRealEstateLoansMember		1us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_OtherRealEstateLoansMember
Pre-modification outstanding recorded investment	283us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_OtherRealEstateLoansMember		2,426us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_OtherRealEstateLoansMember
Post-modification outstanding recorded investment	$ 344us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_OtherRealEstateLoansMember		$ 2,426us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember / us-gaap_MortgageLoansOnRealEstateDescriptionLoanCategoryAxis = fxnc_OtherRealEstateLoansMember

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

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Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

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