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Securities - Summary of Amortized Costs and Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 90,141us-gaap_AvailableForSaleSecuritiesAmortizedCost	$ 83,494us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross Unrealized Gains	1,090us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax	704us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Gross Unrealized (Losses)	(376)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	(906)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Fair Value	90,855us-gaap_AvailableForSaleSecurities	83,292us-gaap_AvailableForSaleSecurities
U.S. Agency and Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	74,540us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	67,462us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
Gross Unrealized Gains	650us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	374us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
Gross Unrealized (Losses)	(354)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	(807)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
Fair Value	74,836us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	67,029us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,600us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember	16,031us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
Gross Unrealized Gains	435us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember	325us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
Gross Unrealized (Losses)	(22)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember	(99)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
Fair Value	16,013us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember	16,257us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
Corporate Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember	1us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Gross Unrealized Gains	5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember	5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Fair Value	$ 6us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember	$ 6us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember

X

- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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-Publisher FASB

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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

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-Section S99

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