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

This is threshold for designation to impaired status for loan relationship that are non accrual, significantly past due, display credit weakness or are TDR.

+ 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

Amount of allowance for credit losses related to receivables which had been measured under a general allowance for credit losses methodology that are now identified as troubled debt restructurings.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: 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=82895541&loc=d3e18845-111554

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

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Balance Type:	na
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- Definition

The amount of financing receivables reclassified to held for sale during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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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=114873765&loc=SL114874048-224260

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

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- Details

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Data Type:	na
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- Details

Name:	us-gaap_LoanRestructuringModificationAxis=nksh_InterestOnlyPaymentsMember
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- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
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Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nksh_CommercialRealEstateOwnerOccupiedMember
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Data Type:	na
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- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nksh_CommercialNonRealEstateSegmentMember
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Data Type:	na
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- Details

Name:	us-gaap_LoanRestructuringModificationAxis=us-gaap_ContractualInterestRateReductionMember
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Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_LoanRestructuringModificationAxis=nksh_InterestMethodChangedFromVariableToFixedMember
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Data Type:	na
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- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=nksh_ConsumerNonRealEstatePortfolioSegmentMember
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Data Type:	na
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- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
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Data Type:	na
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Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SpecialMentionMember
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Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=nksh_ClassifiedExcludingImpairedMember
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Data Type:	na
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- 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, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Threshold for Designation to Impaired Status			$ 250
Interest and Fee Income, Loans and Leases, Total	$ 8,005	$ 7,473	$ 23,159	$ 22,379
Threshold Period for Considering Loans as Special Mention or Classified			75 days
Financing Receivable, Reclassification to Held-for-sale			$ 0
Financing Receivable, Modifications, Recorded Investment	$ 10,700	$ 7,964	$ 10,700	$ 7,964	$ 7,897
Financing Receivable, Modifications, Number of Contracts	2	4	6	7
Allowance for Credit Losses, Change in Method of Calculating Impairment	$ 0	$ 0	$ 0	$ 0
Financing Receivable, Modifications, Subsequent Default, Number of Contracts			0	0
Residential Portfolio Segment [Member] \| Investor Owned Residential Real Estate [Member]
Financing Receivable, Modifications, Number of Contracts	2		2
Residential Portfolio Segment [Member] \| Investor Owned Residential Real Estate [Member] \| Interest Only Payments [Member]
Financing Receivable, Modifications, Number of Contracts	1		1
Commercial Real Estate Portfolio Segment [Member] \| Commercial Real Estate, Owner Occupied [Member]
Financing Receivable, Modifications, Number of Contracts			2	1
Commercial Real Estate Portfolio Segment [Member] \| Commercial Real Estate, Owner Occupied [Member] \| Interest Only Payments [Member]
Financing Receivable, Modifications, Number of Contracts			2
Commercial Non Real Estate Segment [Member]
Financing Receivable, Modifications, Number of Contracts		2		4
Commercial Non Real Estate Segment [Member] \| Contractual Interest Rate Reduction [Member]
Financing Receivable, Modifications, Number of Contracts				3
Commercial Non Real Estate Segment [Member] \| Interest Method Changed from Variable to Fixed [Member]
Financing Receivable, Modifications, Number of Contracts				1
Consumer Non Real Estate Portfolio Segment [Member] \| Automobile Loan [Member]
Financing Receivable, Modifications, Number of Contracts		2		2
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