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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, 2022 | Dec. 31, 2021
Ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees and costs |  | 0.96% | 0.96%
Ratio of net charge-offs to average loans, net of unearned income and deferred fees and costs | [1] | 0.02% | 0.05%
[1] | Net charge-offs are on an annualized basis.

X
- 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: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- 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: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration