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Note 6 - Fair Value Measurements - Level 3 Fair Value Measurements (Details) - Valuation, Market Approach [Member]

Jun. 30, 2021

Dec. 31, 2020

Measurement Input, Discount Rate [Member] | Minimum [Member]

Impaired loans

^[1]

0.0550

OREO

^[2]

0.0000

0.0000

Measurement Input, Discount Rate [Member] | Maximum [Member]

Impaired loans

^[1]

0.0650

OREO

^[2]

0.3376

0.0766

Measurement Input, Discount Rate [Member] | Weighted Average [Member]

Impaired loans

^[1]

(0.0578)

Measurement Input, Cost to Sell [Member] | Minimum [Member]

OREO

^[2]

0.0600

0.0400

Measurement Input, Cost to Sell [Member] | Maximum [Member]

OREO

^[2]

0.0620

0.0923

[1]	Unobservable inputs were weighted by the relative fair value of the impaired loans.
[2]	Discounts were weighted by the relative appraised value of the OREO properties.

X

- Definition

Value of input used to measure impaired loans.

+ References

No definition available.

+ Details

Name:	nksh_ImpairedLoansMeasurementInput
Namespace Prefix:	nksh_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

+ Details

Name:	us-gaap_OtherRealEstateOwnedMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputCostToSellMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: