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Commitments and Unfunded Credits - Additional Information (Detail) $ in Thousands | Dec. 31, 2017 USD ($)
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Commitments and Contingencies Disclosure [Abstract] |
Locked-rate commitments to originate mortgage loans | $ 2,800
Loans held for sale | 438
Amount on deposit in banks exceeded the insurance limits of the Federal Deposit Insurance Corporation | $ 11

X
- Definition Access deposit in other commercial banks. + References No definition available. + Details
Name: | fxnc_AccessDepositInOtherCommercialBanks
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of locked rate commitments for loans held for sale. + References No definition available. + Details
Name: | fxnc_AmountOfLockedRateCommitmentsForLoansHeldForSale
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of locked rate commitments for mortgage loans. + References No definition available. + Details
Name: | fxnc_AmountOfLockedRateCommitmentsForMortgageLoans
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration