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Funds Restrictions and Reserve Balance - Additional Information (Detail) (USD $) | Dec. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Aggregate amount of unrestricted funds | $ 253,000 |
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 | $ 1,400,000 | $ 3,000,000

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

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

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