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Note 10 - Restrictions on Dividends (Details Textual) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2025 | Dec. 31, 2024
Restrictions on Dividends [Line Items] |  |
SEC Schedule, 12-04, Cash Dividends Paid to Registrant, Consolidated Subsidiaries | $ 14,000 | $ 20,000
Period of Restriction on Retained Net Income Without Prior Approval (Year) | 2 years |

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- Definition Period of restrictions on banking or savings institution's ability to pay dividends. + References No definition available. + Details
Name: | nksh_PeriodOfRestrictionOnRetainedNetIncomeWithoutPriorApproval
Namespace Prefix: | nksh_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Restrictions on dividends. + References No definition available. + Details
Name: | nksh_RestrictionsOnDividendsLineItems
Namespace Prefix: | nksh_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents disclosure of the aggregate cash dividends paid to the entity by consolidated subsidiaries. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 942 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477314/942-235-S99-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480738/235-10-S50-4 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04(b)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480678/235-10-S99-3 + Details
Name: | us-gaap_CashDividendsPaidToParentCompanyByConsolidatedSubsidiaries
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration