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Note 15 - Fair Value Measurements - Level 3 Fair Value Measurements (Details) - Valuation, Market Approach [Member] | Dec. 31, 2020 | Dec. 31, 2019
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Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |
Discount rate | [1] | 0.0550 | 0.0550
Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |
Discount rate | [1] | 0.0650 | 0.0650
Measurement Input, Discount Rate [Member] | Weighted Average [Member] |  |  |
Discount rate | [1] | (0.0578) | (0.0577)
Measurement Input, Cost to Sell [Member] | Maximum [Member] |  |  |
Level 3 Fair Value Measurements | [2] | 0.0923 | 0.0600
Measurement Input, Cost to Sell [Member] | Weighted Average [Member] |  |  |
Level 3 Fair Value Measurements | [2] | (0.0454) | (0.0068)
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Minimum [Member] |  |  |
Level 3 Fair Value Measurements | [2] | 0.0000 | 0.0000
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Maximum [Member] |  |  |
Level 3 Fair Value Measurements | [2] | 0.0766 | 0.4517
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Weighted Average [Member] |  |  |
Level 3 Fair Value Measurements | [2] | (0.0062) | (0.0128)
[1] | Unobservable inputs were weighted by the relative fair value of the impaired loans.
[2] | Discounts were weighted by the relative appraised value of the OREO properties

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- Definition Value of input used to measure impaired loans. + References No definition available. + Details
Name: | nksh_ImpairedLoansMeasurementInput
Namespace Prefix: | nksh_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Definition Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
Name: | us-gaap_OtherRealEstateOwnedMeasurementInput
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputCostToSellMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_MeasurementInputTypeAxis=nksh_MeasurementInputDiscountForLackOfMarketabilityAndAgeOfAppraisalMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |