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Benefit Plans - Additional Information (Detail) (USD $) | 3 Months Ended
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Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |
Number of highest-paid consecutive years for benefits | 5 years
Defined Benefit Pension Plan [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Minimum eligibility for full-time employees, age | 21 years
Amount contributed to pension plan, Expected in future | 0 us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInCurrentFiscalYear / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Amount contributed to pension plan | 0 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PensionPlansDefinedBenefitMember
Defined Benefit Pension Plan [Member] | Minimum [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Period of service credit by employee | 1 year

X
- Definition Defined benefit pension plan minimum eligible age of employee. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Number of highest paid consecutive periods of service for calculation of average compensation related to benefits. + 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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- Definition Period of service credit by employee. + References No definition available. + Details
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Namespace Prefix: | fxnc_
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Balance Type: | na
Period Type: | duration

X
- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan in the current remaining fiscal period. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Balance Type: |
Period Type: |