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Deposits - Additional Information (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Banking and Thrift [Abstract] |  |
Time deposits, in denominations of $250 thousand or more | $ 14,100 | $ 10,100
Brokered deposits | $ 550 | $ 601

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