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Capital Requirements - Additional Information (Detail) | 9 Months Ended
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Sep. 30, 2016
Regulatory Capital Requirements [Abstract] |
Capital conservation buffer period | 4 years

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- Definition Capital conservation buffer phase period. + References No definition available. + Details
Name: | fxnc_CapitalConservationBufferPhasePeriod
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_RegulatoryCapitalRequirementsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration