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Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Nonperforming Assets (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014 | Mar. 31, 2014
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Nonperforming Assets [Abstract] |  |  |  |  |  |
Nonaccrual loans | $ 3,102 nksh_LoanReceivableNonaccrual |  | $ 3,999 nksh_LoanReceivableNonaccrual |  | $ 5,071 nksh_LoanReceivableNonaccrual |
Restructured loans in nonaccrual | 6,123 nksh_NonaccrualRestructuredLoans |  | 5,288 nksh_NonaccrualRestructuredLoans |  | 1,007 nksh_NonaccrualRestructuredLoans |
Total nonperforming loans | 9,225 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus | [1] | 9,287 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus | [1] | 6,078 us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus |
Other real estate owned, net | 4,573 us-gaap_RealEstateAcquiredThroughForeclosure |  | 4,744 us-gaap_RealEstateAcquiredThroughForeclosure |  | 4,901 us-gaap_RealEstateAcquiredThroughForeclosure |
Total nonperforming assets | $ 13,798 nksh_NonperformingAssets |  | $ 14,031 nksh_NonperformingAssets |  | $ 10,979 nksh_NonperformingAssets |
Ratio of nonperforming assets to loans, net of unearned income and deferred fees, plus other real estate owned | 2.20%nksh_AssetsRatioOfNonperformingAssetsToLoansNetOfUnearnedIncomeAndDeferredFeesPlusOreo |  | 2.30%nksh_AssetsRatioOfNonperformingAssetsToLoansNetOfUnearnedIncomeAndDeferredFeesPlusOreo |  | 1.85%nksh_AssetsRatioOfNonperformingAssetsToLoansNetOfUnearnedIncomeAndDeferredFeesPlusOreo |
Ratio of allowance for loan losses to nonperforming loans(1) | 89.51%nksh_RatioOfAllowanceForLoanLossesToNonperformingLoans | [2] | 88.97%nksh_RatioOfAllowanceForLoanLossesToNonperformingLoans | [2] | 136.51%nksh_RatioOfAllowanceForLoanLossesToNonperformingLoans | [2]
[1] | Only classes with past-due or nonaccrual loans are shown.
[2] | The Company defines nonperforming loans as nonaccrual loans. Loans 90 days or more past due and still accruing and accruing restructured loans are excluded.

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- Definition Reflects the percentage derived from dividing the nonperforming assets by total loans net of unearned income deferred fees plus other real estate owned. + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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- Definition Recorded investment in non-restructured loans that are on nonaccrual status as of the balance sheet date. + References No definition available. + Details
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Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Refers to restructured loans in nonaccrual. + References No definition available. + Details
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Balance Type: | debit
Period Type: | instant

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- Definition Represents financing receivables that are in nonaccrual status and other real estate owned. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Details
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- Definition Refers to ratio of allowance for loan losses to nonperforming loans. + References No definition available. + Details
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- Definition Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5111-111524 + Details
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- Definition Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4437-111522 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 28 -Article 12 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph c -Subparagraph Schedule III -Article 5 + Details
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