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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, 2020 | Sep. 30, 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(2) | [1] | 1.05% | 1.01% | 0.94%
Ratio of net charge-offs to average loans, net of unearned income and deferred fees and costs(1) | [2] | 0.07% | 0.09% | 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 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 1.13%.
[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
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