Note 6 - Fair Value Measurements - Impaired Loans and Other Real Estate Owned Measured at Fair Value on Nonrecurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Impaired loans net of valuation allowance		$ 970
Other real estate owned net of valuation allowance	$ 957	1,553
Fair Value, Inputs, Level 3 [Member]
Impaired loans net of valuation allowance		970
Other real estate owned net of valuation allowance	$ 957	$ 1,553

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of impaired loans net of valuation allowance.

No definition available.

Name:	nksh_ImpairedLoansNetFairValueDisclosure
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant