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Commitments and Unfunded Credits - Additional Information (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Locked-rate commitments to originate mortgage loans	$ 2,800
Loans held for sale	438
Amount on deposit in banks exceeded the insurance limits of the Federal Deposit Insurance Corporation	$ 11

X

- Definition

Access deposit in other commercial banks.

+ References

No definition available.

+ Details

Name:	fxnc_AccessDepositInOtherCommercialBanks
Namespace Prefix:	fxnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of locked rate commitments for loans held for sale.

+ References

No definition available.

+ Details

Name:	fxnc_AmountOfLockedRateCommitmentsForLoansHeldForSale
Namespace Prefix:	fxnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Amount of locked rate commitments for mortgage loans.

+ References

No definition available.

+ Details

Name:	fxnc_AmountOfLockedRateCommitmentsForMortgageLoans
Namespace Prefix:	fxnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- References

No definition available.

+ Details

Name:	us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration