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Benefit Plans (Details Textual) (USD $) In Thousands, unless otherwise specified | 9 Months Ended
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Sep. 30, 2012
Benefit Plans (Textual) [Abstract] |
Amount contributed to pension plan | $ 509
Period of service credit by employee | 1 year

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- Details
Name: | fxnc_BenefitPlansTextualAbstract
Namespace Prefix: | fxnc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Period of service credit by employee. + References No definition available. + Details
Name: | fxnc_PeriodOfServiceCreditByEmployee
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan in the current remaining fiscal period. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanEstimatedFutureEmployerContributionsInCurrentFiscalYear
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant