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Dividend Reinvestment Plan - Additional Information (Detail) - shares | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Equity [Abstract] |  |
Period preceding the dividend payment date | 10 days |
Issuance of common stock dividend reinvestment plan, shares (in shares) | 3,035 | 3,949

X
- Definition Number of consecutive trading days for purchase of common share. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_EquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of shares issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the shareholders to reinvest dividends paid to them by the entity on new issues of stock by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 + Details
Name: | us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration