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Note 7 - Fair Value Measurements - Level 3 Fair Value Measurements (Details) - Valuation, Market Approach [Member] | Jun. 30, 2019 | Dec. 31, 2018
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Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |
Discount rate |  | 0.05 | 0.055
Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |
Discount rate |  | 0.0725 | 0.0725
Measurement Input, Discount Rate [Member] | Weighted Average [Member] |  |  |
Discount rate |  | 0.056 | 0.0605
Measurement Input, Cost to Sell [Member] | Minimum [Member] |  |  |
Level 3 Fair Value Measurements | [1] | 0 | 0
Measurement Input, Cost to Sell [Member] | Maximum [Member] |  |  |
Level 3 Fair Value Measurements |  | 0.06 | 0.06
Measurement Input, Cost to Sell [Member] | Weighted Average [Member] |  |  |
Level 3 Fair Value Measurements |  | 0.0004 | 0.0012
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Minimum [Member] |  |  |
Level 3 Fair Value Measurements |  | 0 | 0
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Maximum [Member] |  |  |
Level 3 Fair Value Measurements |  | 0.1067 | 0.5005
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Weighted Average [Member] |  |  |
Level 3 Fair Value Measurements |  | 0.0008 | 0.0145
[1] | The Company markets other real estate owned both independently and with local realtors. Properties marketed by realtors are discounted by selling costs. Properties that the Company markets independently are not discounted by selling costs.

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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: |