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Note 6 - Fair Value Measurements - Quantitative Information About Level 3 Fair Value Measurements (Details) - Fair Value, Nonrecurring [Member] - Collateral Dependent Loans [Member] $ in Thousands	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Assets, Fair Value	$ 812	$ 533
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value	$ 812	$ 533
Fair Value, Inputs, Level 3 [Member] | Measurement Input, Discount Rate [Member]
Collateral dependent loans	0.065	0.065

X

- Definition

Value of input used to measure collateral dependent loans.

+ References

No definition available.

+ Details

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Namespace Prefix:	fxnc_
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- Definition

Fair value portion of asset recognized for present right to economic benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
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-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

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- Details

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- Details

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- Details

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- Details

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