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Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment | $ 16,311 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | [1],[2] | $ 16,654 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | [1] | $ 13,540 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | [1]
Interest income recognized | 473 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod | [2] | 267 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod |  | 9 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod |
Closed-end First Liens [Member] | Consumer Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment | 555 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndFirstLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2] | 364 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndFirstLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2] | 873 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndFirstLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2]
Interest income recognized | 31 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndFirstLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [2] | 3 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndFirstLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [2] | 2 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndFirstLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [2]
Closed-end Junior Liens [Member] | Consumer Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment | 249 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndJuniorLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2] | 280 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndJuniorLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2] | 234 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndJuniorLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2]
Interest income recognized | 16 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndJuniorLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [2] | 9 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ClosedEndJuniorLiensMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [2] |  | [2]
Investor-owned Residential Real Estate [Member] | Consumer Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment | 77 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_InvestorOwnedResidentialRealEstateMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2] | 131 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_InvestorOwnedResidentialRealEstateMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2] |  |
Interest income recognized | 5 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_InvestorOwnedResidentialRealEstateMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [2] | 6 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_InvestorOwnedResidentialRealEstateMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [2] |  |
Multifamily Real Estate [Member] | Commercial Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment | 2,773 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_MultifamilyRealEstateMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [1],[2] | 4,172 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_MultifamilyRealEstateMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [1],[2] | 1,466 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_MultifamilyRealEstateMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [1],[2]
Interest income recognized |  | [2] |  | [2] | 5 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_MultifamilyRealEstateMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [2]
Commercial Real Estate, Owner Occupied [Member] | Commercial Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment | 5,836 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [1],[2] | 5,265 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [1],[2] | 4,806 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [1],[2]
Interest income recognized | 203 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [2] | 136 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [2] | 1 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOwnerOccupiedMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [2]
Commercial Real Estate, Other [Member] | Commercial Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment | 6,114 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOtherMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [1],[2] | 3,369 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOtherMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [1],[2] |  |
Interest income recognized | 175 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOtherMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [2] | 110 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialRealEstateOtherMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialRealEstateLoansMember | [2] |  |
Commercial and Industrial [Member] | Commercial Non Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment | 707 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialAndIndustrialMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialNonRealEstateMember | [1],[2] | 117 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialAndIndustrialMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialNonRealEstateMember | [1],[2] |  |
Interest income recognized | 43 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialAndIndustrialMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialNonRealEstateMember | [2] | 3 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialAndIndustrialMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialNonRealEstateMember | [2] |  |
Construction Residential [Member] | Real Estate Construction [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment |  |  | 40 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ConstructionResidentialMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_RealEstateConstructionMember | [1],[2] | 1,171 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ConstructionResidentialMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_RealEstateConstructionMember | [1],[2]
Interest income recognized |  |  |  | [2] |  | [2]
Construction Other [Member] | Real Estate Construction [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment |  |  | 2,885 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ConstructionOtherMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_RealEstateConstructionMember | [1],[2] | 4,290 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ConstructionOtherMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_RealEstateConstructionMember | [1],[2]
Interest income recognized |  |  |  | [2] | 1 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_ConstructionOtherMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_RealEstateConstructionMember | [2]
Automobile [Member] | Consumer Non Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment |  |  | 31 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_AutomobileMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerNonRealEstateMember | [1],[2] | 4 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_AutomobileMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerNonRealEstateMember | [1],[2]
Interest income recognized |  |  |  | [2] |  | [2]
Equity Lines [Member] | Consumer Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment |  |  |  |  | 101 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_EquityLinesMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerRealEstateMember | [1],[2]
Interest income recognized |  |  |  |  |  | [2]
Commercial [Member] | Commercial Non Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment |  |  |  |  | 570 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_CommercialMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_CommercialNonRealEstateMember | [1],[2]
Interest income recognized |  |  |  |  |  | [2]
Other Consumer Loans [Member] | Consumer Non Real Estate [Member] |  |  |  |  |  |
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Impaired Loans, Average Investment and Interest Income Recognized [Line Items] |  |  |  |  |  |
Average recorded investment |  |  |  |  | 25 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = nksh_OtherConsumerLoansMember / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = nksh_ConsumerNonRealEstateMember | [1],[2]
Interest income recognized |  |  |  |  |  | [2]
[1] | Recorded investment is net of charge-offs and interest paid while a loan is in nonaccrual status.
[2] | Only classes with impaired loans are shown.

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