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Note 5 - Allowance for Credit Losses on Loans and Nonperforming Assets - Ratios of Allowance for Loan Losses (Details) | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023
Ratio of ACLL to the end of period loans, net of deferred fees and costs | 1.04% | 1.06%
Ratio of net charge-offs to average loans, net of deferred fees and costs | 0.03% | 0.02%

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

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- Definition Percentage of allowance for credit loss to total financing receivable outstanding. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-K (SK) -Number 229 -Section 1405 -Paragraph a -Subparagraph (1) -Publisher SEC + Details
Name: | srt_FinancingReceivableAllowanceForCreditLossToOutstandingPercent
Namespace Prefix: | srt_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant