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Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans - Ratios of Allowance for Loan Losses (Details) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees and costs(1) | [1] | 1.10% | 0.94%
Ratio of net charge-offs to average loans, net of unearned income and deferred fees and costs |  | 0.05% | 0.09%
[1] | The ratio of the allowance for loan losses to the end of period loans, net of unearned income and deferred fees and costs at December 31, 2020 includes government-guaranteed SBA PPP loans, which do not require an allowance for loan losses. Excluding the PPP loans, the ratio would be 1.16%.

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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