Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Ratios for Allowance for Loan Losses	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Ratios for Allowance for Loan Losses [Abstract]
Ratio of allowance for loan losses to the end of period loans, net of unearned income and deferred fees	1.36%	1.38%
Ratio of net charge-offs to average loans, net of unearned income and deferred fees	0.27%nksh_RatioOfNetChargeOffsToAverageLoans	0.28%nksh_RatioOfNetChargeOffsToAverageLoans

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