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Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Interest and Fee Income, Loans and Leases (in Dollars) | $ 31,217 us-gaap_InterestAndFeeIncomeLoansAndLeases | $ 32,819 us-gaap_InterestAndFeeIncomeLoansAndLeases | $ 35,354 us-gaap_InterestAndFeeIncomeLoansAndLeases
Threshold Period for Considering Loans As Special Mention Or Classified |  | 75 days |
Financing Receivable, Modifications, Number of Contracts | 5 us-gaap_FinancingReceivableModificationsNumberOfContracts2 | 6 us-gaap_FinancingReceivableModificationsNumberOfContracts2 |
Allowance for Credit Losses, Change in Method of Calculating Impairment (in Dollars) | 206 us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment | 11 us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment |
Financing Receivable, Modifications, Subsequent Default, Number of Contracts | 0 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 | 5 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 |
Financing Receivable, Allowance for Credit Losses, Write-downs (in Dollars) | 1,860 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs | 1,820 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs | 2,953 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Interest Capitalized [Member] | Commercial Real Estate, Non Owner Occupied [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Financing Receivable, Modifications, Number of Contracts | 2 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateNonOwnerOccupiedMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_InterestCapitalizedMember |  |
Interest-only Payments [Member] | Commercial Real Estate, Non Owner Occupied [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Financing Receivable, Modifications, Number of Contracts | 2 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateNonOwnerOccupiedMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_InterestOnlyPaymentsMember |  |
Consumer Real Estate and Commercial Real Estate Loan [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Financing Receivable, Allowance for Credit Losses, Write-downs (in Dollars) |  | 263 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_ConsumerRealEstateAndCommercialRealEstateLoanMember |
Commercial Real Estate [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Financing Receivable, Allowance for Credit Losses (in Dollars) |  | 349 us-gaap_FinancingReceivableAllowanceForCreditLosses / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_CommercialRealEstateLoansMember |
Commercial Real Estate, Non Owner Occupied [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Financing Receivable, Modifications, Interest Only Payment Term | 1 year |  |
Commercial Real Estate [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Financing Receivable, Modifications, Number of Contracts |  | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember |
Financing Receivable, Modifications, Subsequent Default, Number of Contracts |  | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember |
Other Real Estate, Additions (in Dollars) |  | 193 us-gaap_OtherRealEstateAdditions / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember |
Financing Receivable, Allowance for Credit Losses, Write-downs (in Dollars) | 1,201 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | 64 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | 1,589 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember
Consumer Real Estate [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Financing Receivable, Modifications, Number of Contracts |  | 2 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember |
Financing Receivable, Modifications, Subsequent Default, Number of Contracts |  | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember |
Financing Receivable, Allowance for Credit Losses, Write-downs (in Dollars) | 222 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | 256 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | 370 us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down (in Dollars) |  | 71 us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember |
Commercial [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Financing Receivable, Modifications, Number of Contracts |  | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialMember |
Financing Receivable, Modifications, Subsequent Default, Number of Contracts |  | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialMember |
Nonperforming Financing Receivable [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Interest and Fee Income, Loans and Leases (in Dollars) | $ 0 us-gaap_InterestAndFeeIncomeLoansAndLeases / us-gaap_InternalCreditAssessmentAxis = us-gaap_NonperformingFinancingReceivableMember | $ 0 us-gaap_InterestAndFeeIncomeLoansAndLeases / us-gaap_InternalCreditAssessmentAxis = us-gaap_NonperformingFinancingReceivableMember | $ 0 us-gaap_InterestAndFeeIncomeLoansAndLeases / us-gaap_InternalCreditAssessmentAxis = us-gaap_NonperformingFinancingReceivableMember
Special Mention [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Increase in Percentage of Allocation Loans Rated Special Mention |  | 50.00%nksh_IncreaseInPercentageOfAllocationLoansRatedSpecialMention / us-gaap_InternalCreditAssessmentAxis = us-gaap_SpecialMentionMember |
Classified Excluding Impaired [Member] |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items] |  |  |
Increase in Percentage of Allocation Loans Rated Classified |  | 100.00%nksh_IncreaseInPercentageOfAllocationLoansRatedClassified / us-gaap_InternalCreditAssessmentAxis = nksh_ClassifiedExcludingImpairedMember |

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- Definition Amount of allowance for credit losses related to receivables which had been measured under a general allowance for credit losses methodology that are now identified as troubled debt restructurings. + References No definition available. + Details
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- Definition Number of financing receivables that have been modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524 + Details
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- Definition Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524 + Details
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- Definition The aggregate interest and fee income generated by: (1) loans the Entity has positive intent and ability to hold for the foreseeable future, or until maturity or payoff, including commercial and consumer loans, whether domestic or foreign, which may consist of: (a) industrial and agricultural; (b) real estate; and (c) real estate construction loans; (d) trade financing; (e) lease financing; (f) home equity lines-of-credit; (g) automobile and other vehicle loans; and (h) credit card and other revolving-type loans and (2) loans and leases held-for-sale which may include mortgage loans, direct financing, and sales-type leases. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.1) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 1 -Article 9 + Details
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- Definition Additions of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This excludes real estate assets taken in settlement of troubled loans through surrender or foreclosure. + References No definition available. + Details
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