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Income Taxes - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Deferred tax asset will not be realized "More likely than not" defined percentage	50.00%
Pre-tax loss reported period	2 years

X

- Definition

Cumulative pretax loss position period.

+ References

No definition available.

+ Details

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

X

- Definition

More likely than not percentage.

+ References

No definition available.

+ Details

Name:	fxnc_MoreLikelyThanNotPercentage
Namespace Prefix:	fxnc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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