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Note 7 - Fair Value Measurements - Impaired Loans and Other Real Estate Owned Measured at Fair Value on Nonrecurring Basis (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Impaired loans net of valuation allowance | $ 1,248 | $ 1,303
Other real estate owned net of valuation allowance | 2,214 | 2,817
Fair Value, Inputs, Level 1 [Member] |  |
Impaired loans net of valuation allowance |  | 0
Other real estate owned net of valuation allowance | 0 |
Fair Value, Inputs, Level 2 [Member] |  |
Impaired loans net of valuation allowance |  | 0
Other real estate owned net of valuation allowance | 0 |
Fair Value, Inputs, Level 3 [Member] |  |
Impaired loans net of valuation allowance | 1,248 | 1,303
Other real estate owned net of valuation allowance | $ 2,214 | $ 2,817

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- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of impaired loans net of valuation allowance. + References No definition available. + Details
Name: | nksh_ImpairedLoansNetFairValueDisclosure
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of other real estate owned by bank. + References No definition available. + Details
Name: | nksh_OtherRealEstateFairValueDisclosure
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |