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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 ($)	Dec. 31, 2022 USD ($)
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.

X

- 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

X

- Definition

Value of input used to measure impaired loans.

+ References

No definition available.

+ Details

Name:	fxnc_ImpairedLoansMeasurementInput
Namespace Prefix:	fxnc_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- 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

X

- 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

X

- Details

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Namespace Prefix:
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- Details

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Period Type:

X

- 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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