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Note 7 - Fair Value Measurements - Level 3 Fair Value Measurements (Details) - Valuation, Market Approach [Member] | Mar. 31, 2020 | Dec. 31, 2019
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Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |
Discount rate | [1] | 0.055 | 0.055
Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |
Discount rate | [1] | 0.065 | 0.065
Measurement Input, Discount Rate [Member] | Weighted Average [Member] |  |  |
Discount rate | [1] | (0.0577) | (0.0577)
Measurement Input, Cost to Sell [Member] | Minimum [Member] |  |  |
Level 3 Fair Value Measurements | [2],[3] | 0 | 0
Measurement Input, Cost to Sell [Member] | Maximum [Member] |  |  |
Level 3 Fair Value Measurements | [2] | 0.1 | 0.06
Measurement Input, Cost to Sell [Member] | Weighted Average [Member] |  |  |
Level 3 Fair Value Measurements | [2] | (0.0065) | (0.0068)
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Minimum [Member] |  |  |
Level 3 Fair Value Measurements | [2] | 0 | 0
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Maximum [Member] |  |  |
Level 3 Fair Value Measurements | [2] | 0.04 | 0.4517
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Weighted Average [Member] |  |  |
Level 3 Fair Value Measurements | [2] | (0.0033) | (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.
[3] | The appraised value is discounted by selling costs if the OREO property is listed with a realtor and if appraised value exceeds the list price, less estimated selling costs. Selling costs do not discount appraised value if the Company markets the OREO property independently or if the OREO property is listed with a realtor and the list price less estimated selling costs exceeds appraised value.

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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: |