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Note 7 - Fair Value Measurements - Level 3 Fair Value Measurements (Details) - Valuation, Market Approach [Member] | Sep. 30, 2018 | Dec. 31, 2017
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Measurement Input, Discount Rate [Member] | Minimum [Member] |  |
Discount rate | 0.055 | 0.055
Measurement Input, Discount Rate [Member] | Maximum [Member] |  |
Discount rate | 0.0725 | 0.1325
Measurement Input, Discount Rate [Member] | Weighted Average [Member] |  |
Discount rate | 0.0604 | 0.0592
Measurement Input, Cost to Sell [Member] | Minimum [Member] |  |
Level 3 Fair Value Measurements | 0.05 | 0.02
Measurement Input, Cost to Sell [Member] | Maximum [Member] |  |
Level 3 Fair Value Measurements | 0.0681 | 0.0601
Measurement Input, Cost to Sell [Member] | Weighted Average [Member] |  |
Level 3 Fair Value Measurements | 0.0575 | 0.0472
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Minimum [Member] |  |
Level 3 Fair Value Measurements | 0.0319 | 0.0168
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Maximum [Member] |  |
Level 3 Fair Value Measurements | 0.378 | 0.6833
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Weighted Average [Member] |  |
Level 3 Fair Value Measurements | 0.134 | 0.1107

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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=99377789&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
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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: |