Increase in the percentage of allocation for loans rated classified.

No definition available.

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- Definition

Increase in the percentage of allocation for loans rated special mention.

+ References

No definition available.

+ Details

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- Definition

Represents number of loan modifications for COVID-19 related accommodations.

+ References

No definition available.

+ Details

Name:	nksh_NumberOfLoanModificationsForCOVID19RelatedAccommodations
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Data Type:	xbrli:integerItemType
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Period Type:	instant

- Definition

Represents principal balance of loans with COVID-19 related accommodations,

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents ratio of allowance for loan losses, excluding PPP loans, to the end of period loans, net of unearned income and deferred fees.

+ References

No definition available.

+ Details

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- Definition

Represents the threshold period for considering loans as special mention or classified.

+ References

No definition available.

+ Details

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- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

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Period Type:	duration

- Definition

The aggregate interest and fee income generated by: (1) loans the Entity has positive intent and ability to hold for the foreseeable future, or until maturity or payoff, including commercial and consumer loans, whether domestic or foreign, which may consist of: (a) industrial and agricultural; (b) real estate; and (c) real estate construction loans; (d) trade financing; (e) lease financing; (f) home equity lines-of-credit; (g) automobile and other vehicle loans; and (h) credit card and other revolving-type loans and (2) loans and leases held-for-sale which may include mortgage loans, direct financing, and sales-type leases.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.1)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=nksh_ClassifiedExcludingImpairedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentPerformanceStatusAxis=us-gaap_NonperformingFinancingReceivableMember
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Note 3 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details Textual) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Ratio of Allowance for Loan Losses to End of Period Loans, Excluding PPP Loans			1.13%
Interest and Fee Income, Loans and Leases, Total	$ 8,606	$ 8,548	$ 25,491	$ 25,277
Threshold Period for Considering Loans as Special Mention or Classified (Day)			75 days
Financing Receivable, Troubled Debt Restructuring	$ 4,292	$ 5,257	$ 4,292	$ 5,257	$ 4,940
Financing Receivable, Modifications, Number of Contracts	0	1	0	1
Number of Loan Modifications for COVID-19 Related Accommodations	361		361
Principal Balance of Loans with COVID-19 Related Accommodations	$ 160,230		$ 160,230
Consumer Real Estate Portfolio Segment [Member]
Financing Receivable, Troubled Debt Restructuring		$ 263		$ 263
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts				7
Special Mention [Member]
Increase in Percentage of Allocation Loans Rated Special Mention			50.00%
Classified Excluding Impaired [Member]
Increase in Percentage of Allocation Loans Rated Classified			100.00%
Nonperforming Financial Instruments [Member]
Interest and Fee Income, Loans and Leases, Total			$ 0	$ 0	$ 0