Commitments and Unfunded Credits - Additional Information (Detail) $ in Thousands	Dec. 31, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Locked-rate commitments to originate mortgage loans	$ 6,400
Loans held for sale	419
Amount on deposit in banks exceeded the insurance limits of the Federal Deposit Insurance Corporation	$ 40

Amount of locked rate commitments for loans held for sale.

No definition available.

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