Note 5 - Allowance for Credit Losses on Loans and Nonperforming Assets (Details Textual)	12 Months Ended
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

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

No definition available.

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