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Fair Value Measurements - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Non recurring fair value adjustments on loans held for sale | $ 0 | $ 0
Period of maturity as classifies as short-term borrowings | 90 days |
Fair value of loan commitments and standby letters of credit | $ 0 | $ 0
Maximum [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Period under consideration for valuation of real estate collateral | 12 months |
Period under consideration for valuation of business equipment | 1 year |
Minimum [Member] |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Period under consideration for valuation of house or building in the process of construction | 1 year |

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- Definition Fair value of loan commitments and standby letters of credit. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Non recurring fair value adjustments on loans held for sale. + References No definition available. + Details
Name: | fxnc_NonrecurringFairValueAdjustmentsOnLoansHeldForSale
Namespace Prefix: | fxnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Minimum period of appraisal of property to consider different method to measure fair value. + References No definition available. + Details
Name: | fxnc_PeriodOfAppraisalOfPropertyToConsiderDifferentMethodToMeasureFairValue
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Period of maturity as classifies as short term borrowing. + 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 Maximum period used to determine value of business equipment. + References No definition available. + Details
Name: | fxnc_PeriodUsedToDetermineValueOfBusinessEquipment
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Maximum period used to determine value of real estate collateral. + References No definition available. + Details
Name: | fxnc_PeriodUsedToDetermineValueOfRealEstateCollateral
Namespace Prefix: | fxnc_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |