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Note 19 - Fair Value Measurements - Quantitative Information About Level 3 Fair Value Measurements (Details) - Fair Value, Nonrecurring [Member] $ in Thousands | Dec. 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
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Collateral Dependent Loans [Member] |  |  |
Assets, Fair Value |  | $ 533 |
Other Real Estate Owned [Member] |  |  |
Assets, Fair Value |  |  | $ 184
Impaired Loans [Member] |  |  |
Assets, Fair Value |  |  | $ 197
Fair Value, Inputs, Level 3 [Member] |  |  |
Impaired loans, net |  |  | 1.97
Fair Value, Inputs, Level 3 [Member] | Measurement Input, Cost to Sell [Member] |  |  |
Other real estate owned | [1] |  | 0.10
Fair Value, Inputs, Level 3 [Member] | Collateral Dependent Loans [Member] |  |  |
Assets, Fair Value |  | $ 533 |
Fair Value, Inputs, Level 3 [Member] | Collateral Dependent Loans [Member] | Measurement Input, Discount Rate [Member] |  |  |
Collateral dependent loans |  | 0.10 |
Fair Value, Inputs, Level 3 [Member] | Other Real Estate Owned [Member] |  |  |
Assets, Fair Value |  |  | $ 184
Fair Value, Inputs, Level 3 [Member] | Other Real Estate Owned [Member] | Valuation Technique, Property Appraisals [Member] |  |  |
Assets, Fair Value |  |  | 184
Fair Value, Inputs, Level 3 [Member] | Impaired Loans [Member] |  |  |
Assets, Fair Value |  |  | $ 197
Fair Value, Inputs, Level 3 [Member] | Impaired Loans [Member] | Measurement Input, Cost to Sell [Member] |  |  |
Impaired loans, net |  |  | 0.065
[1] | Unobservable inputs were weighted by the relative fair value of the instruments.

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- Definition Value of input used to measure collateral dependent loans. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Definition Value of input used to measure impaired loans. + References No definition available. + Details
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Namespace Prefix: | fxnc_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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