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Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | $ 17,130 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |  | $ 16,605 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets |
Cash [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 737 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 1,319 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Cash [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 737 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember |  | 1,319 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CashMember |
Equity Securities US Companies [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 8,729 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = nksh_EquitySecuritiesUSCompaniesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 8,091 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = nksh_EquitySecuritiesUSCompaniesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Equity Securities US Companies [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 8,729 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = nksh_EquitySecuritiesUSCompaniesMember |  | 8,091 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = nksh_EquitySecuritiesUSCompaniesMember |
Equity Securities International Companies [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 274 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = nksh_EquitySecuritiesInternationalCompaniesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 137 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = nksh_EquitySecuritiesInternationalCompaniesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Equity Securities International Companies [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 274 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = nksh_EquitySecuritiesInternationalCompaniesMember |  | 137 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = nksh_EquitySecuritiesInternationalCompaniesMember |
Equity Funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 1,861 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | [1] | 2,490 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | [1]
Equity Funds [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 1,861 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember | [1] | 2,490 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquityFundsMember | [1]
US Government Corporations and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 107 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 358 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
US Government Corporations and Agencies Securities [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 107 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember |  | 358 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember |
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 365 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 398 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
US States and Political Subdivisions Debt Securities [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 365 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 398 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Corporate Debt Securities [Member] | External Credit Rating, Investment Grade [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 5,057 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [2] | 3,812 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | [2]
Corporate Debt Securities [Member] | External Credit Rating, Investment Grade [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 5,057 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember | [2] | 3,812 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember | [2]
Fair Value, Inputs, Level 1 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | 11,601 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 12,037 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Fair Value, Inputs, Level 2 [Member] |  |  |  |
Note 8 - Employee Benefit Plans (Details) - Fair Value Measurements [Line Items] |  |  |  |
Pension plan’s assets | $ 5,529 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | $ 4,568 us-gaap_DefinedBenefitPlanFairValueOfPlanAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
[1] | This category comprises actively managed equity funds invested in large-cap and mid-cap U.S. companies.
[2] | This category represents investment grade bonds of U.S. issuers from diverse industries.

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- Definition Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=29635418&loc=d3e12355-114930 + Details
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