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Note 16 - Fair Value Measurements (Details) - Estimated Fair Values and Related Carrying Amounts of Financial Instruments (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Inputs, Level 1 [Member]
Financial assets:
Cash and due from banks	$ 13,283	$ 14,783
Interest-bearing deposits	97,376	96,597
Accrued interest receivable	5,949	6,247
Bank-owned life insurance	21,181	20,523
Financial liabilities:
Deposits	718,327	669,028
Accrued interest payable	92	139
Fair Value, Inputs, Level 2 [Member]
Financial assets:
Securities	343,005	360,661
Restricted securities	1,414	1,689
Mortgage loans held for sale	1,276	2,796
Fair Value, Inputs, Level 3 [Member]
Financial assets:
Loans, net	616,755	570,471
Financial liabilities:
Deposits	247,753	272,820
Reported Value Measurement [Member]
Financial assets:
Cash and due from banks	13,283	14,783
Interest-bearing deposits	97,376	96,597
Securities	347,651	350,354
Restricted securities	1,414	1,689
Mortgage loans held for sale	1,276	2,796
Loans, net	587,463	583,813
Accrued interest receivable	5,949	6,247
Bank-owned life insurance	21,181	20,523
Financial liabilities:
Deposits	960,036	946,766
Accrued interest payable	$ 92	$ 139

X

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of accrued interest payable.

+ References

No definition available.

+ Details

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

X

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of accrued interest receivable.

+ References

No definition available.

+ Details

Name:	nksh_AccruedInterestReceivableFairValueDisclosure
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

For banks and other depository institutions: Includes fair value of cash on hand (currency and coin), cash items in process of collection, noninterest bearing deposits due from other financial institutions (including corporate credit unions), and noninterest bearing balances with the Federal Reserve Banks, Federal Home Loan Banks and central banks.

+ References

No definition available.

+ Details

Name:	nksh_CashAndDueFromBanksFairValueDisclosure
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

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

X

- Details

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

X

- Definition

For banks and other depository institutions (including Federal Reserve Banks, if applicable). Fair value of interest-bearing deposits in other financial institutions for relatively short periods of time including, for example, certificates of deposits, which are presented separately from cash on the balance sheet.

+ References

No definition available.

+ Details

Name:	nksh_InterestBearingDepositsFairValueDisclosure
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents securities which consist of all investments in certain debt and equity securities.

+ References

No definition available.

+ Details

Name:	nksh_SecuritiesFairValueDisclosure
Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Fair value portion of the amount that could be realized under a life insurance contract or contracts owned by the entity, commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Surrender Value

-URI http://asc.fasb.org/extlink&oid=6507153

+ Details

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

X

- Definition

Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

+ References

No definition available.

+ Details

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

X

- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Glossary Loan

-URI http://asc.fasb.org/extlink&oid=6517073

+ Details

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

X

- Definition

Fair value portion of mortgage loans held-for-sale.

+ References

No definition available.

+ Details

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

X

- Definition

The aggregate value of all restricted investments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 320

-Section S99

-Paragraph 4

-Subparagraph (SX 210.12-12C.9(c))

-URI http://asc.fasb.org/extlink&oid=6955306&loc=d3e611225-123010



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 946

-SubTopic 320

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-12.6(c))

-URI http://asc.fasb.org/extlink&oid=6955306&loc=d3e611133-123010



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 12C

-Paragraph 9

-Subparagraph c

-Article 12



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 12

-Paragraph 6

-Subparagraph c

-Article 12

+ Details

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