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Capital Requirements - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2016
Regulatory Capital Requirements [Abstract]
Capital conservation buffer period	4 years

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- Definition

Capital conservation buffer phase period.

+ References

No definition available.

+ Details

Name:	fxnc_CapitalConservationBufferPhasePeriod
Namespace Prefix:	fxnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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- References

No definition available.

+ Details

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