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Securities - Additional Information (Detail) (USD $) | 12 Months Ended
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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 | 29 fxnc_NumberOfSecuritiesInUnrealizedLossPosition | 7 fxnc_NumberOfSecuritiesInUnrealizedLossPosition |
Number of obligations of state and political subdivisions in an unrealized loss position | 21 fxnc_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 | $ 22,300,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds | $ 4,600,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds | $ 36,300,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds
Gross realized gains on securities available for sale | 787,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains | 0 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains | 1,300,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Gross realized losses on securities available for sale | 91,000 us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses | 0 us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses | 0 us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses
Securities book value | 4,500,000 us-gaap_AvailableForSaleSecuritiesRestricted | 3,100,000 us-gaap_AvailableForSaleSecuritiesRestricted |
Impairment recognized | $ 0 fxnc_ImpairmentLossesRelatedToFederalHomeLoanStock |  |

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- Definition 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 + Details
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- Definition 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 + Details
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