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Note 5 - Allowance for Credit Losses on Loans and Nonperforming Assets (Details Textual) | 12 Months Ended
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Dec. 31, 2024 USD ($) Securities | Dec. 31, 2023 USD ($) Securities
Maximum Loan amount indicated as risk graded | $ 400,000 |
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts | Securities | 0 | 0
Commercial Real Estate Portfolio Segment [Member] | Commercial Real Estate, Owner Occupied [Member] | Special Mention [Member] | Valuation Technique, Fair Value of Collateral Method [Member] |  |
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment | $ 0 | $ 347,000

X
- Definition Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment. + References No definition available. + Details
Name: | nksh_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default. + References No definition available. + Details
Name: | nksh_FinancingReceivableTroubledDebtRestructuringSubsequentDefaultNumberOfContracts
Namespace Prefix: | nksh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum loan amount indicated as risk graded. + References No definition available. + Details
Name: | nksh_MaximumLoanAmountIndicatedAsRiskGraded
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nksh_CommercialRealEstateOwnerOccupiedMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ValuationTechniqueAxis=nksh_ValuationTechniqueFairValueOfCollateralMethodMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |