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Income Taxes - Additional Information (Detail) | 12 Months Ended
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Dec. 31, 2014
Income Tax Disclosure [Abstract] |
Deferred tax asset will not be realized "More likely than not" defined percentage | 50.00%fxnc_MoreLikelyThanNotPercentage
Pre-tax loss reported period | 2 years

X
- Definition Cumulative pretax loss position period. + References No definition available. + Details
Name: | fxnc_CumulativePretaxLossPositionPeriod
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition More likely than not percentage. + References No definition available. + Details
Name: | fxnc_MoreLikelyThanNotPercentage
Namespace Prefix: | fxnc_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_IncomeTaxDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration