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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, 2017 | Dec. 31, 2016
Ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees and costs | 1.19% | 1.28%
Ratio of net charge-offs to average loans, net of unearned income and deferred fees and costs | 0.08% | 0.26%

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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