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Securities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Obligation Securities	Dec. 31, 2013 Securities	Dec. 31, 2012
Amortized Cost and Fair Value Debt Securities [Abstract]
Number of U.S. agency and mortgage-backed securities	29fxnc_NumberOfSecuritiesInUnrealizedLossPosition	7fxnc_NumberOfSecuritiesInUnrealizedLossPosition
Number of obligations of state and political subdivisions in an unrealized loss position	21fxnc_NumberObligationOfStateAndPoliticalSubdivisionsInUnrealizedLossPosition
Percentage of investment portfolio	100.00%fxnc_PercentageOfInvestmentPortfolio	100.00%fxnc_PercentageOfInvestmentPortfolio
Weighted-average period of re-pricing of portfolio	3 years 10 months 24 days	4 years 6 months
Proceeds from maturities, calls, and principal payments of securities available for sale	$ 22,300,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds	$ 4,600,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds	$ 36,300,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds
Gross realized gains on securities available for sale	787,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains	0us-gaap_AvailableForSaleSecuritiesGrossRealizedGains	1,300,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Gross realized losses on securities available for sale	91,000us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses	0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses	0us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Securities book value	4,500,000us-gaap_AvailableForSaleSecuritiesRestricted	3,100,000us-gaap_AvailableForSaleSecuritiesRestricted
Impairment recognized	$ 0fxnc_ImpairmentLossesRelatedToFederalHomeLoanStock

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Impairment losses related to federal home loan stock.

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Number obligation of state and political subdivisions in unrealized loss position.

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Number of securities in unrealized loss position.

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Percentage of investment portfolio.

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Weighted average period of pricing of portfolio.

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This item represents the gross profit realized on the sale of debt or equity securities 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

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

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The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563

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The carrying amount of securities categorized neither as held-to-maturity nor trading which serve as security for borrowings or other arrangements.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 6

-Article 9



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph b

-Article 4



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5086-111524



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX210.4-08(b))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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