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Note 8 - Employee Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Note 8 - Employee Benefit Plans (Details) [Line Items] |  |  |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent | 100.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent |  |
Defined Contribution Plan, Employer Discretionary Contribution Amount | $ 273 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount | $ 282 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount | $ 279 us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount
Age Attained to Be Eligible for ESOP | 21 years |  |
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP | 100 us-gaap_EmployeeStockOwnershipPlanESOPCashContributionsToESOP | 275 us-gaap_EmployeeStockOwnershipPlanESOPCashContributionsToESOP | 370 us-gaap_EmployeeStockOwnershipPlanESOPCashContributionsToESOP
Employee Stock Ownership Plan (ESOP), Number of Allocated Shares (in Shares) | 246,370 us-gaap_EmployeeStockOwnershipPlanESOPNumberOfAllocatedShares |  |
Age of Employees Required to Diversify Allocated ESOP Shares | 55 years |  |
Years of Plan Participation Required to Diversify Allocated ESOP Shares | 10 years |  |
Percentage of Allocated ESOP Shares That Can Be Diversified | 50.00%nksh_PercentageOfAllocatedEsopSharesThatCanBeDiversified |  |
Accrued Expenses for Salary Continuation Plan | $ 246 nksh_AccruedExpensesForSalaryContinuationPlan | $ 194 nksh_AccruedExpensesForSalaryContinuationPlan | $ 151 nksh_AccruedExpensesForSalaryContinuationPlan
Equity Securities [Member] |  |  |
Note 8 - Employee Benefit Plans (Details) [Line Items] |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 65.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_EquitySecuritiesMember |  |
Fixed Income Securities [Member] |  |  |
Note 8 - Employee Benefit Plans (Details) [Line Items] |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 35.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_FixedIncomeSecuritiesMember |  |
Hedge Funds [Member] |  |  |
Note 8 - Employee Benefit Plans (Details) [Line Items] |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_HedgeFundsMember |  |
Private Equity Funds [Member] |  |  |
Note 8 - Employee Benefit Plans (Details) [Line Items] |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_PrivateEquityFundsMember |  |
Real Estate [Member] |  |  |
Note 8 - Employee Benefit Plans (Details) [Line Items] |  |  |
Defined Benefit Plan, Target Plan Asset Allocations | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocations / us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis = us-gaap_RealEstateMember |  |

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- Definition Liability for agreement to continue an employee's salary after termination of employment including retirement, death, disability, or other event. For example, a company may agree to continue a disabled employee's salary for six months. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Age of an employee to be eligible for the Employee Stock Ownership Plan (ESOP). + References No definition available. + Details
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- Definition Age required to diversify allocated Employee Stock Ownership Plan (ESOP) shares. + References No definition available. + Details
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- Details
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- Definition Percentage of allocated Employee Stock Ownership Plan shares that can diversified. + References No definition available. + Details
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- Definition Years of plan participation required to diversify Employee Stock Ownership Plan (ESOP). + References No definition available. + Details
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- Definition Target allocation of investments of this type to total plan assets presented on a weighted average basis. + 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 (d)(i) -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 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 + Details
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- Definition Amount of discretionary contributions made by an employer to a defined contribution plan. + References No definition available. + Details
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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Data Type: | num:percentItemType
Balance Type: | na
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- Definition This item represents the amount of cash contributions during the period made by the entity to the Employee Stock Ownership Plan (ESOP). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 + Details
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- Definition The shares or units in an ESOP trust that have been assigned to individual participant accounts based on a known formula. IRS rules require allocations to be nondiscriminatory generally based on compensation, length of service, or a combination of both. For any particular participant such shares or units may be vested, unvested, or partially vested. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929 + Details
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- Details
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