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Note 16 - Fair Value Measurements (Details) - Level 3 Fair Value Measurements | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Discounted Appraised Value [Member] | Impaired loans [Member] | Minimum [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Selling cost range | 10.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1],[2] | 0.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1]
Selling cost weighted average | (10.00%)nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1],[2] | 0.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1]
Discounted Appraised Value [Member] | Impaired loans [Member] | Maximum [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Selling cost range |  |  | 10.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1]
Selling cost weighted average |  |  | (10.00%)nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1]
Discounted Appraised Value [Member] | Impaired loans [Member] | Weighted Average [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Selling cost range |  |  | 5.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1]
Selling cost weighted average |  |  | (5.00%)nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1]
Discounted Appraised Value [Member] | Impaired loans [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Valuation technique | Discounted appraised value |  | Discounted appraised value |
Discounted rate range | Discounted appraised value |  | Discounted appraised value |
Discounted Appraised Value [Member] | Other Real Estate Owned [Member] | Minimum [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Valuation technique | 0% |  | 0% |
Selling cost range | 0.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1] | 0.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1]
Selling cost weighted average | 0.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1] | 0.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_MinimumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember | [1]
Discounted rate range | 0% |  | 0% |
Discounted Appraised Value [Member] | Other Real Estate Owned [Member] | Maximum [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Valuation technique | 48.77% |  | 29.60% |
Selling cost range | 11.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |  | 10.00%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |
Selling cost weighted average | (11.00%)nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |  | (10.00%)nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_MaximumMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |
Discounted rate range | 48.77% |  | 29.60% |
Discounted Appraised Value [Member] | Other Real Estate Owned [Member] | Weighted Average [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Selling cost range | 8.60%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |  | 9.83%nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |
Selling cost weighted average | (8.60%)nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |  | (9.83%)nksh_FairValueInputsSellingCostRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |
Discounted rate weighted average | (20.81%)us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |  | (8.66%)us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByAssetClassAxis = nksh_OtherRealEstateOwnedMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_DiscountedAppraisedValueMember |
Discounted Appraised Value [Member] | Other Real Estate Owned [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Valuation technique | Discounted appraised value |  | Discounted appraised value |
Discounted rate range | Discounted appraised value |  | Discounted appraised value |
Present Value of Cash Flows [Member] | Impaired loans [Member] | Minimum [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Valuation technique | 5.88% |  | 6.25% |
Discounted rate range | 5.88% |  | 6.25% |
Present Value of Cash Flows [Member] | Impaired loans [Member] | Maximum [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Valuation technique | 9.50% |  | 9.50% |
Discounted rate range | 9.50% |  | 9.50% |
Present Value of Cash Flows [Member] | Impaired loans [Member] | Weighted Average [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Discounted rate weighted average | (6.15%)us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_PresentValueOfCashFlowsMember |  | (6.75%)us-gaap_FairValueInputsDiscountRate / us-gaap_FairValueByAssetClassAxis = nksh_ImpairedLoansMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / us-gaap_ValuationTechniqueAxis = nksh_PresentValueOfCashFlowsMember |
Present Value of Cash Flows [Member] | Impaired loans [Member] |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |
Valuation technique | Present value of cash flows |  | Present value of cash flows |
Discounted rate range | Present value of cash flows |  | Present value of cash flows |
[1] | Impaired loans that are collateral-dependent are valued using the fair value of collateral. The valuation is discounted for selling costs if repayment of the loan is dependent on the sale of the collateral. If repayment will come from rental income of the property, the valuation is not discounted for selling costs.
[2] | Only one loan was valued using the collateral method as of December 31, 2014.

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- Definition Refers to the rate of selling cost. + References No definition available. + Details
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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition Description of the inputs and valuation technique(s) used to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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