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Funds Restrictions and Reserve Balance - Additional Information (Detail) (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Aggregate amount of unrestricted funds | $ 20,500,000 fxnc_AggregateAmountOfUnrestrictedFundsTransferredFromBank |
First Bank [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Aggregate amounts of daily average required balances of reserve against its deposits | $ 613,000 fxnc_AggregateAmountOfDailyAverageDeposit / dei_LegalEntityAxis = fxnc_FirstBankMember | $ 1,400,000 fxnc_AggregateAmountOfDailyAverageDeposit / dei_LegalEntityAxis = fxnc_FirstBankMember

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- Definition Aggregate amount of daily average deposit. + References No definition available. + Details
Name: | fxnc_AggregateAmountOfDailyAverageDeposit
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Aggregate amount of unrestricted funds transferred from bank. + References No definition available. + Details
Name: | fxnc_AggregateAmountOfUnrestrictedFundsTransferredFromBank
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- Details
Name: | dei_LegalEntityAxis=fxnc_FirstBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |