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Securities - Summary of Investments in an Unrealized Loss Position that were Temporarily Impaired (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value	$ 7,438us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue	$ 9,392us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
12 months or more - Fair Value	22,103us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue	35,368us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total - Fair Value	29,541us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue	44,760us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Less than 12 months - Unrealized (Loss)		(61)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
12 months or more - Unrealized (Loss)		(845)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Total Unrealized (Loss)	(376)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax	(906)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
U.S. Agency and Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value	7,438us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	8,677us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
12 months or more - Fair Value	21,024us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	32,527us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
Total - Fair Value	28,462us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	41,204us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
Less than 12 months - Unrealized (Loss)	(18)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	(60)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
12 months or more - Unrealized (Loss)	(336)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	(747)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
Total Unrealized (Loss)	(354)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember	(807)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = fxnc_UsAgencyAndMortgageBackedSecuritiesMember
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months - Fair Value		715us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
12 months or more - Fair Value	1,079us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember	2,841us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
Total - Fair Value	1,079us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember	3,556us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
Less than 12 months - Unrealized (Loss)		(1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
12 months or more - Unrealized (Loss)	(22)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember	(98)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember
Total Unrealized (Loss)	$ (22)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember	$ (99)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember

X

- 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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563



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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

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-Section 50

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Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563



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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563



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+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563



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