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Note 3 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans - Ratios of Allowance for Loan Losses (Details) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019
Ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees and costs |  | 0.99% | 1.02% | 0.94%
Ratio of net charge-offs to average loans, net of unearned income and deferred fees and costs(1) | [1] | 0.06% | 0.13% | 0.09%
[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