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Funds Restrictions and Reserve Balance - Additional Information (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Aggregate amount of unrestricted funds	$ 20,500,000fxnc_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	$ 613,000fxnc_AggregateAmountOfDailyAverageDeposit / dei_LegalEntityAxis = fxnc_FirstBankMember	$ 1,400,000fxnc_AggregateAmountOfDailyAverageDeposit / dei_LegalEntityAxis = fxnc_FirstBankMember

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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

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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: