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Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans - Ratios of Allowance for Loan Losses (Details) | 9 Months Ended | 12 Months Ended
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Sep. 30, 2016 | Sep. 30, 2015 | Dec. 31, 2015
Ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees |  | 1.30% | 1.30% | 1.34%
Ratio of net charge-offs to average loans, net of unearned income and deferred fees(1) | [1] | 0.25% | 0.19% | 0.32%
[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