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Note 7 - Fair Value Measurements - Level 3 Fair Value Measurements (Details) - Valuation, Market Approach [Member] | Mar. 31, 2018 | Dec. 31, 2017
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Measurement Input, Discount Rate [Member] | Minimum [Member] |  |
Discount rate | 5.5 | 5.5
Measurement Input, Discount Rate [Member] | Maximum [Member] |  |
Discount rate | 13.25 | 13.25
Measurement Input, Discount Rate [Member] | Weighted Average [Member] |  |
Discount rate | 5.92 | 5.92
Measurement Input, Cost to Sell [Member] | Minimum [Member] |  |
Level 3 Fair Value Measurements | 5 | 2
Measurement Input, Cost to Sell [Member] | Maximum [Member] |  |
Level 3 Fair Value Measurements | 6.01 | 6.01
Measurement Input, Cost to Sell [Member] | Weighted Average [Member] |  |
Level 3 Fair Value Measurements | 5.14 | 4.72
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Minimum [Member] |  |
Level 3 Fair Value Measurements | 1.68 | 1.68
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Maximum [Member] |  |
Level 3 Fair Value Measurements | 86.03 | 68.33
Measurement Input, Discount for Lack of Marketability and Age of Appraisal [Member] | Weighted Average [Member] |  |
Level 3 Fair Value Measurements | 13.35 | 11.07

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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
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: |