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Note 11 - Minimum Regulatory Capital Requirement (Details Textual) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
National Bank of Blacksburg [Member]
Banking Regulation, Risk-Weighted Assets, Actual	$ 932,364	$ 816,962

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- Definition

Amount of risk-weighted assets as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826

+ Details

Name:	us-gaap_RiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Details

Name:	dei_LegalEntityAxis=nksh_NationalBankOfBlacksburgMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: