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Securities - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	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	19fxnc_NumberOfSecuritiesInUnrealizedLossPosition	29fxnc_NumberOfSecuritiesInUnrealizedLossPosition
Number of obligations of state and political subdivisions in an unrealized loss position	2fxnc_NumberObligationOfStateAndPoliticalSubdivisionsInUnrealizedLossPosition	7fxnc_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	$ 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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