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Fair Value Measurements - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2013 | Dec. 31, 2012
Fair Value Disclosures [Abstract] |  |
Non recurring fair value adjustments on loans held for sale | $ 0 | $ 0
Period under consideration for valuation of real estate collateral | 1 year |
Period under consideration for valuation of house or building in the process of construction | 1 year |
Period under consideration for valuation of business equipment | 1 year |
Period of maturity as classifies as short term borrowing | 90 days |
Fair value of loan commitments and standby letters of credit | $ 0 | $ 0

X
- Definition Fair value of loan commitments and standby letters of credit. + References No definition available. + Details
Name: | fxnc_FairValueOfLoanCommitmentsAndStandbyLettersOfCredit
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Maximum period used to determine value of business equipment. + References No definition available. + Details
Name: | fxnc_MaximumPeriodUsedToDetermineValueOfBusinessEquipment
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum period used to determine value of real estate collateral. + References No definition available. + Details
Name: | fxnc_MaximumPeriodUsedToDetermineValueOfRealEstateCollateral
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Minimum period of appraisal of property to consider different method to measure fair value. + References No definition available. + Details
Name: | fxnc_MinimumPeriodOfAppraisalOfPropertyToConsiderDifferentMethodToMeasureFairValue
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Non recurring fair value adjustments on loans held for sale. + References No definition available. + Details
Name: | fxnc_NonrecurringFairValueAdjustmentsOnLoansHeldForSale
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Period of maturity as classifies as short term borrowing. + References No definition available. + Details
Name: | fxnc_PeriodOfMaturityAsClassifiesAsShortTermBorrowing
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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