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Note 8 - Employee Benefit Plans (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent	100.00%
Defined Contribution Plan, Employer Discretionary Contribution Amount	$ 394	$ 379	$ 364
Age Attained to be Eligible for ESOP (Year)	21 years
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP	$ 300	300	300
Employee Stock Ownership Plan (ESOP), Number of Allocated Shares (in shares)	184,503
Age of Employees Required to Diversify Allocated ESOP Shares (Year)	55 years
Years of Plan Participation Required to Diversify Allocated ESOP Shares (Year)	10 years
Percentage of Allocated ESOP Shares That Can be Diversified	50.00%
Accrued Expenses for Salary Continuation Plan	$ 304	$ 270	$ 255
Equity Securities [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	65.00%
Fixed Income Securities [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	35.00%
Hedge Funds [Member]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%

X

- Definition

Liability for agreement to continue an employee's salary after termination of employment including retirement, death, disability, or other event. For example, a company may agree to continue a disabled employee's salary for six months.

+ References

No definition available.

+ Details

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Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Age of an employee to be eligible for the Employee Stock Ownership Plan (ESOP).

+ References

No definition available.

+ Details

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Namespace Prefix:	nksh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Age required to diversify allocated Employee Stock Ownership Plan (ESOP) shares.

+ References

No definition available.

+ Details

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Namespace Prefix:	nksh_
Data Type:	xbrli:durationItemType
Balance Type:	na
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X

- Definition

Percentage of allocated Employee Stock Ownership Plan shares that can diversified.

+ References

No definition available.

+ Details

Name:	nksh_PercentageOfAllocatedEsopSharesThatCanBeDiversified
Namespace Prefix:	nksh_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Years of plan participation required to diversify Employee Stock Ownership Plan (ESOP).

+ References

No definition available.

+ Details

Name:	nksh_YearsOfPlanParticipationRequiredToDiversifyAllocatedEsopShares
Namespace Prefix:	nksh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(i)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

This item represents the amount of cash contributions during the period made by the entity to the Employee Stock Ownership Plan (ESOP).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 40
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109244661&loc=d3e17540-113929

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The shares or units in an ESOP trust that have been assigned to individual participant accounts based on a known formula. IRS rules require allocations to be nondiscriminatory generally based on compensation, length of service, or a combination of both. For any particular participant such shares or units may be vested, unvested, or partially vested.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 40
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109244661&loc=d3e17540-113929

+ Details

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X

- Details

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X

- Details

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Period Type:

X

- Details

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