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Deposits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Deposits [Abstract] |  |
Time deposits, in denominations of $250 thousand or more | $ 11.4 fxnc_TimeDeposits250000OrMore | $ 11.8 fxnc_TimeDeposits250000OrMore
Brokered deposits | $ 1.9 fxnc_BrokeredDeposits | $ 5.1 fxnc_BrokeredDeposits

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- Definition Total brokered deposits. + References No definition available. + Details
Name: | fxnc_BrokeredDeposits
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Time Deposits 250000 Or More + References No definition available. + Details
Name: | fxnc_TimeDeposits250000OrMore
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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