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Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Ratios for Allowance for Loan Losses | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Ratios for Allowance for Loan Losses [Abstract] |  |  |  |  |  |
Ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees | 1.33%nksh_RatioOfAllowanceForLoanLossesToEndOfPeriodLoans |  | 1.41%nksh_RatioOfAllowanceForLoanLossesToEndOfPeriodLoans |  | 1.36%nksh_RatioOfAllowanceForLoanLossesToEndOfPeriodLoans |
Ratio of net charge-offs to average loans, net of unearned income and deferred fees(1) | 0.14%nksh_RatioOfNetChargeOffsToAverageLoans | [1] | 0.02%nksh_RatioOfNetChargeOffsToAverageLoans | [1] | 0.27%nksh_RatioOfNetChargeOffsToAverageLoans | [1]
[1] | Net charge-offs are on an annualized basis.

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- Definition Represents ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees. + References No definition available. + Details
Name: | nksh_RatioOfAllowanceForLoanLossesToEndOfPeriodLoans
Namespace Prefix: | nksh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents ratio of net charge-offs to average loans, net of unearned income and deferred fees. + References No definition available. + Details
Name: | nksh_RatioOfNetChargeOffsToAverageLoans
Namespace Prefix: | nksh_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | nksh_RatiosForAllowanceForLoanLossesAbstract
Namespace Prefix: | nksh_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration