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Note 10 - Restrictions on Dividends (Details Textual) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
SEC Schedule, 12-04, Cash Dividends Paid to Registrant, Consolidated Subsidiaries | $ 22,000 | $ 28,556 | $ 9,419
Period of Restriction on Retained Net Income Without Prior Approval (Year) | 2 years |  |
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments without Regulatory Approval | $ 0 |  |

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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 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 -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04(b)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e24072-122690 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of dividends that can be declared or paid without approval of regulatory agency. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant