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Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Restructured Loans that Defaulted (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts | 0 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 | 5 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 |
Recorded Investment |  | $ 1,327 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 | [1]
Consumer Real Estate [Member] | Closed-end First Liens [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts |  | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndFirstLiensMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_ConsumerRealEstateMember |
Recorded Investment |  | 24 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndFirstLiensMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_ConsumerRealEstateMember | [1]
Consumer Real Estate [Member] | Closed-end Junior Liens [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts |  | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndJuniorLiensMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_ConsumerRealEstateMember |
Recorded Investment |  | 81 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndJuniorLiensMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_ConsumerRealEstateMember | [1]
Commercial Real Estate [Member] | Commercial Real Estate, Owner Occupied [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts |  | 2 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_CommercialRealEstateLoansMember |
Recorded Investment |  | 834 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_CommercialRealEstateLoansMember | [1]
Commercial Non Real Estate [Member] | Commercial and Industrial [Member] |  |  |
Financing Receivable, Modifications [Line Items] |  |  |
Number of Contracts |  | 1 us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_CommercialNonRealEstateMember |
Recorded Investment |  | $ 388 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialAndIndustrialMember / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = nksh_CommercialNonRealEstateMember | [1]
[1] | Recorded investment at the time the default occurred.

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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 Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, 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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