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Deposits - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Banking and Thrift [Abstract]
Time deposits, in denominations of $250 thousand or more	$ 14,100	$ 10,100
Brokered deposits	$ 550	$ 601

X

- 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

X

- 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

X

- References

No definition available.

+ Details

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