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Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Troubled Debt Restructurings (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Commercial Real Estate, Owner Occupied [Member] | Commercial Real Estate [Member] |  |
Commercial Real Estate |  |
Number of contracts | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember
Pre-modification outstanding principal balance | $ 1,007 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | $ 184 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember
Post-modification outstanding principal balance | 907 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | 209 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember
Commercial Real Estate [Member] |  |
Commercial Real Estate |  |
Number of contracts | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateLoansMember | 1 us-gaap_FinancingReceivableModificationsNumberOfContracts2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateLoansMember
Pre-modification outstanding principal balance | 1,007 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateLoansMember | 184 us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateLoansMember
Post-modification outstanding principal balance | $ 907 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateLoansMember | $ 209 us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2 / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateLoansMember

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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 Post-modification amount of investment in financing receivables 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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