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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
Dec. 31, 2014
Dec. 31, 2013
Financing Receivable, Modifications [Line Items]    
Number of Contracts 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 5us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Recorded Investment   $ 1,327us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 [1]
Consumer Real Estate [Member] | Closed-end First Liens [Member]    
Financing Receivable, Modifications [Line Items]    
Number of Contracts   1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= nksh_ClosedEndFirstLiensMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nksh_ConsumerRealEstateMember
Recorded Investment   24us-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   1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= nksh_ClosedEndJuniorLiensMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nksh_ConsumerRealEstateMember
Recorded Investment   81us-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   2us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= nksh_CommercialRealEstateOwnerOccupiedMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nksh_CommercialRealEstateLoansMember
Recorded Investment   834us-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   1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= nksh_CommercialAndIndustrialMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nksh_CommercialNonRealEstateMember
Recorded Investment   $ 388us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis
= nksh_CommercialAndIndustrialMember
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nksh_CommercialNonRealEstateMember		 [1]
[1] Recorded investment at the time the default occurred.