Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method [Line Items]
Allowance for loan losses, individually evaluated for impairment	$ 236	$ 282
Allowance for loan losses, collectively evaluated for impairment	8,021	7,981
Allowance for loan losses	8,257	8,263	8,297	8,227
Loans individually evaluated for impairment	14,994	15,121
Loans collectively evaluated for impairment	608,018	591,198
Total loans	623,012	606,319
Real Estate Construction [Member]
Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method [Line Items]
Allowance for loan losses, collectively evaluated for impairment	467	612
Allowance for loan losses	467	612	850	863
Loans collectively evaluated for impairment	41,647	45,562
Total loans	41,647	45,562
Consumer Real Estate [Member]
Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method [Line Items]
Allowance for loan losses, individually evaluated for impairment	10	14
Allowance for loan losses, collectively evaluated for impairment	1,610	1,648
Allowance for loan losses	1,620	1,662	1,814	1,697
Loans individually evaluated for impairment	809	819
Loans collectively evaluated for impairment	144,891	146,221
Total loans	145,700	147,040
Commercial Real Estate [Member]
Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method [Line Items]
Allowance for loan losses, individually evaluated for impairment	216	258
Allowance for loan losses, collectively evaluated for impairment	3,495	3,279
Allowance for loan losses	3,711	3,537	3,694	3,685
Loans individually evaluated for impairment	13,513	13,624
Loans collectively evaluated for impairment	308,325	297,138
Total loans	321,838	310,762
Commercial Non Real Estate [Member]
Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method [Line Items]
Allowance for loan losses, individually evaluated for impairment	10	10
Allowance for loan losses, collectively evaluated for impairment	1,406	1,465
Allowance for loan losses	1,416	1,475	861	989
Loans individually evaluated for impairment	672	678
Loans collectively evaluated for impairment	31,510	32,734
Total loans	32,182	33,412
Public Sector and IDA [Member]
Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method [Line Items]
Allowance for loan losses, collectively evaluated for impairment	433	327
Allowance for loan losses	433	327	202	132
Loans collectively evaluated for impairment	53,441	41,361
Total loans	53,441	41,361
Consumer Non Real Estate [Member]
Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method [Line Items]
Allowance for loan losses, collectively evaluated for impairment	576	602
Allowance for loan losses	576	602	529	576
Loans collectively evaluated for impairment	28,204	28,182
Total loans	28,204	28,182
Unallocated [Member]
Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans and Allowance for Loan Losses by Evaluation Method [Line Items]
Allowance for loan losses, collectively evaluated for impairment	34	48
Allowance for loan losses	$ 34	$ 48	$ 347	$ 285

Name:	nksh_Note4AllowanceforLoanLossesNonperformingAssetsandImpairedLoansDetailsLoansandAllowanceforLoanLossesbyEvaluationMethodLineItems
Namespace Prefix:	nksh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ 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 (g)

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

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ 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 (h)

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

+ Details

Name:	us-gaap_FinancingReceivableCollectivelyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ 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 (h)

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

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.7(d))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph d

-Article 9

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (g)

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

Reference 4: 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

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (b)(1)(ii)

-URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 6

-Section L

-Subsection 1

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4428-111522

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nksh_CommercialNonRealEstateMember
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_ConsumerNonRealEstateMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=nksh_PublicSectorAndIdaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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