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Note 3 - Loans and Allowance for Credit Losses - Allowance for Credit Losses on Unfunded Commitments (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Balance	$ 10,262	$ 9,094	$ 9,094
Provision (recovery) for credit losses	581	1,312	1,242
Balance	10,579	10,328	10,262
Unfunded Loan Commitment [Member]
Balance	251	259	259
Provision (recovery) for credit losses	37	(25)
Balance	$ 288	241	$ 251
Unfunded Loan Commitment [Member] | Frontier Community Bank [Member]
FCB acquisition		$ 7

X

- Definition

Amount of increase in allowance for credit loss on financing receivable from loan acquired from business combination or acquisition.

+ References

No definition available.

+ Details

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Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of allowance for credit loss on financing receivable. Excludes allowance for financing receivable covered under loss sharing agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
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-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478524/942-220-S99-1

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Period Type: