Period of interest only payments to be made on a loan modified under a troubled debt restructuring.

No definition available.

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

- Definition

Increase in the percentage of allocation for loans rated classified.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Details

Name:	nksh_Note5AllowanceforLoanLossesNonperformingAssetsandImpairedLoansDetailsLineItems
Namespace Prefix:	nksh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- 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

Name:	us-gaap_AllowanceForCreditLossesChangeInMethodOfCalculatingImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- 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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Reflects the amount of the write-down related to a troubled debt restructuring.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554

+ Details

Name:	us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.1)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 1

-Article 9

+ Details

Name:	us-gaap_InterestAndFeeIncomeLoansAndLeases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Additions of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This excludes real estate assets taken in settlement of troubled loans through surrender or foreclosure.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nksh_CommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nksh_CommercialRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nksh_CommercialRealEstateNonOwnerOccupiedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nksh_ConsumerRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableTroubledDebtRestructuringAxis=nksh_CommercialRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableTroubledDebtRestructuringAxis=nksh_ConsumerRealEstateAndCommercialRealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableTroubledDebtRestructuringAxis=nksh_InterestCapitalizedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableTroubledDebtRestructuringAxis=nksh_InterestOnlyPaymentsMember
Namespace Prefix:
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_InternalCreditAssessmentAxis=us-gaap_NonperformingFinancingReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Interest and Fee Income, Loans and Leases (in Dollars)	$ 31,217	$ 32,819	$ 35,354
Threshold Period for Considering Loans As Special Mention Or Classified		75 days
Financing Receivable, Modifications, Number of Contracts	5	6
Allowance for Credit Losses, Change in Method of Calculating Impairment (in Dollars)	206	11
Financing Receivable, Modifications, Subsequent Default, Number of Contracts	0	5
Financing Receivable, Allowance for Credit Losses, Write-downs (in Dollars)	1,860	1,820	2,953
Interest Capitalized [Member] \| Commercial Real Estate, Non Owner Occupied [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Financing Receivable, Modifications, Number of Contracts	2
Interest-only Payments [Member] \| Commercial Real Estate, Non Owner Occupied [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Financing Receivable, Modifications, Number of Contracts	2
Consumer Real Estate and Commercial Real Estate Loan [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Financing Receivable, Allowance for Credit Losses, Write-downs (in Dollars)		263
Commercial Real Estate [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Financing Receivable, Allowance for Credit Losses (in Dollars)		349
Commercial Real Estate, Non Owner Occupied [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Financing Receivable, Modifications, Interest Only Payment Term	1 year
Commercial Real Estate [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Financing Receivable, Modifications, Number of Contracts		1
Financing Receivable, Modifications, Subsequent Default, Number of Contracts		1
Other Real Estate, Additions (in Dollars)		193
Financing Receivable, Allowance for Credit Losses, Write-downs (in Dollars)	1,201	64	1,589
Consumer Real Estate [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Financing Receivable, Modifications, Number of Contracts		2
Financing Receivable, Modifications, Subsequent Default, Number of Contracts		1
Financing Receivable, Allowance for Credit Losses, Write-downs (in Dollars)	222	256	370
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down (in Dollars)		71
Commercial [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Financing Receivable, Modifications, Number of Contracts		1
Financing Receivable, Modifications, Subsequent Default, Number of Contracts		1
Nonperforming Financing Receivable [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Interest and Fee Income, Loans and Leases (in Dollars)	$ 0	$ 0	$ 0
Special Mention [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Increase in Percentage of Allocation Loans Rated Special Mention		50.00%
Classified Excluding Impaired [Member]
Note 5 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) [Line Items]
Increase in Percentage of Allocation Loans Rated Classified		100.00%