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Rights Offering of Common Stock - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014
Equity [Abstract] |
Sale of common shares under rights offering | 1,945,815 fxnc_StockIssuedDuringPeriodSharesRightsExercised
Rights exercised by existing shareholders | 1,520,815 fxnc_NumberOfRightsToPurchaseCommonStockExercisedDuringPeriod
Subscription price of share | $ 4.00 us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Rights purchased by standby investors | 425,000 fxnc_NumberOfAdditionalRightsToPurchaseCommonStockExercisedDuringPeriod
Proceeds from rights issued, net of offering costs | $ 7.6 fxnc_ProceedsFromSaleOfCommonStockInRightsOffering

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- Definition Number of additional rights to purchase common stock exercised during period. + References No definition available. + Details
Name: | fxnc_NumberOfAdditionalRightsToPurchaseCommonStockExercisedDuringPeriod
Namespace Prefix: | fxnc_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of rights to purchase common stock exercised during period. + References No definition available. + Details
Name: | fxnc_NumberOfRightsToPurchaseCommonStockExercisedDuringPeriod
Namespace Prefix: | fxnc_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Proceeds from sale of common stock in rights offering. + References No definition available. + Details
Name: | fxnc_ProceedsFromSaleOfCommonStockInRightsOffering
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Stock issued during period shares rights exercised. + References No definition available. + Details
Name: | fxnc_StockIssuedDuringPeriodSharesRightsExercised
Namespace Prefix: | fxnc_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Exercise price per share or per unit of warrants or rights outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(i)(4)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
Name: | us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix: | us-gaap_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
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Period Type: | duration