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Securities - Additional Information (Detail) (USD $) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 Obligation Securities | Dec. 31, 2014 Obligation Securities
Amortized Cost and Fair Value Debt Securities [Abstract] |  |
Number of U.S. agency and mortgage-backed securities | 19 fxnc_NumberOfSecuritiesInUnrealizedLossPosition | 29 fxnc_NumberOfSecuritiesInUnrealizedLossPosition
Number of obligations of state and political subdivisions in an unrealized loss position | 2 fxnc_NumberObligationOfStateAndPoliticalSubdivisionsInUnrealizedLossPosition | 7 fxnc_NumberObligationOfStateAndPoliticalSubdivisionsInUnrealizedLossPosition
Percentage of investment portfolio | 100.00%fxnc_PercentageOfInvestmentPortfolio | 100.00%fxnc_PercentageOfInvestmentPortfolio
Weighted-average period of re-pricing of portfolio | 3 years 8 months 12 days | 3 years 10 months 24 days
Impairment recognized | $ 0 fxnc_ImpairmentLossesRelatedToFederalHomeLoanStock |

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