Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans - Ratios of Allowance for Loan Losses (Details)	12 Months Ended
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%

Represents ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees.

No definition available.

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Namespace Prefix:	nksh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration