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Benefit Plans - Additional Information (Detail) (USD $) | 9 Months Ended
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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

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- Definition Defined benefit pension plan minimum eligible age of employee. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Number of highest paid consecutive periods of service for calculation of average compensation related to benefits. + References No definition available. + Details
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Namespace Prefix: | fxnc_
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Period Type: | duration

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- Definition Period of service credit by employee. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- 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

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- Details
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Balance Type: | na
Period Type: | duration

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