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Benefit Plans - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014
Defined Benefit Plan Disclosure [Line Items]
Number of highest-paid consecutive years for benefits	5 years
Defined Benefit Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Minimum eligibility for full-time employees, age	21 years
Amount contributed to pension plan, Expected in future	$ 0
Amount contributed to pension plan	$ 0
Defined Benefit Pension Plan [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Period of service credit by employee	1 year

X

- Definition

Defined benefit pension plan minimum eligible age of employee.

+ References

No definition available.

+ Details

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

X

- Definition

Number of highest paid consecutive periods of service for calculation of average compensation related to benefits.

+ References

No definition available.

+ Details

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

X

- Definition

Period of service credit by employee.

+ References

No definition available.

+ Details

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

X

- Definition

For defined benefit pension plans, the actuarial present value of benefits (whether vested or nonvested) attributed by the pension benefit formula to employee service rendered before a specified date and based on employee service and compensation (if applicable) before that date. The accumulated benefit obligation differs from the projected benefit obligation in that it includes no assumption about future compensation levels. For plans with flat-benefit or nonpay-related pension benefit formulas, the accumulated benefit obligation and the projected benefit obligation are the same.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Accumulated Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6503844



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

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

X

- Definition

Amount of employer's best estimate of contributions expected to be paid to the plan in the current remaining fiscal period.

+ References

No definition available.

+ Details

Name:	us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration