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Note 12 - Minimum Regulatory Capital Requirement (Details Textual) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Risk Weighted Assets | $ 808,690 | $ 718,739
National Bank of Blacksburg [Member] |  |
Risk Weighted Assets | $ 805,656 | $ 715,294

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- Definition Amount of risk-weighted assets as calculated in accordance with U.S. Federal regulatory capital standards. + References No definition available. + Details
Name: | us-gaap_RiskWeightedAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | dei_LegalEntityAxis=nksh_NationalBankOfBlacksburgMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |