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v3.3.1.900

Deposits - Additional Information (Detail) - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
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Deposits [Abstract] |  |
Time deposits, in denominations of $250 thousand or more | $ 10,600 | $ 11,400
Brokered deposits | $ 600 | $ 1,900

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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_DepositsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration