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Note 3 - 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, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees and costs(1) | [1] | 0.97% | 1.05% | 1.10%
Ratio of net charge-offs to average loans, net of unearned income and deferred fees and costs(2) | [2] | 0.08% | 0.07% | 0.05%
[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 September 30, 2021, December 31, 2020 and September 30, 2020 includes government-guaranteed SBA PPP loans, which do not require an allowance for loan losses. Excluding the PPP loans, the ratio would be 0.98% at September 30, 2021, 1.16% at December 31, 2020 and 1.13% at September 30, 2020.
[2] | 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
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Namespace Prefix: | nksh_
Data Type: | dtr-types:percentItemType
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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: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration