Note 3 - Securities (Details Textual) $ in Thousands	3 Months Ended
Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Deferred Tax Asset, Debt Securities, Available-for-Sale, Unrealized Loss	$ 18,526	$ 21,644
Number of Temporarily Impaired Securities Greater than Twelve Months	602
Continuous Unrealized Loss Position Aggregate Losses	$ 649,878
Realized Investment Gains (Losses)	12
Available-for-sale Securities Sold, Book Value	17,987
National Bank of Blacksburg [Member] \| Federal Home Loan Bank of Atlanta [Member]
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	646,100
Restricted Stock [Member]
Restricted Investments	$ 929	$ 941

Represents the book value of available-for-sale securities that had been sold.

No definition available.

Name:	nksh_AvailableForSaleSecuritiesSoldBookValue
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of the excess of amortized cost basis over fair value of securities that are in a loss position.

+ References

No definition available.

+ Details

Name:	nksh_ContinuousUnrealizedLossPositionAggregateLosses
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item contains disclosure of the number of investment positions determined to be temporarily impaired in a continuous loss position for twelve months or more.

+ References

No definition available.

+ Details

Name:	nksh_NumberOfTemporarilyImpairedSecuritiesGreaterThanTwelveMonths
Namespace Prefix:	nksh_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, before allocation of valuation allowance, of deferred tax asset attributable to deductible temporary difference from unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482685/740-10-50-6

+ Details

Name:	us-gaap_DeferredTaxAssetsUnrealizedLossesOnAvailableforSaleSecuritiesGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of the collateral pledged in support of federal home loan bank advances outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 470
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480876/942-470-45-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480848/942-470-50-3

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of realized gain (loss) on investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(3)(a))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1

+ Details

Name:	us-gaap_RealizedInvestmentGainsLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 954
-SubTopic 210
-Name Accounting Standards Codification
-Section 45
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480632/954-210-45-4

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 954
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480602/954-210-50-2

+ Details

Name:	us-gaap_RestrictedInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	dei_LegalEntityAxis=nksh_NationalBankOfBlacksburgMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis=srt_FederalHomeLoanBankOfAtlantaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: