Note 15 - Fair Value Measurements - Impaired Loans and Other Real Estate Owned Measured at Fair Value on Nonrecurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Impaired loans net of valuation allowance	$ 970	$ 1,005
Other real estate owned net of valuation allowance	1,553	1,612
Fair Value, Inputs, Level 3 [Member]
Impaired loans net of valuation allowance	970	1,005
Other real estate owned net of valuation allowance	$ 1,553	$ 1,612

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