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Fair Value Measurements, Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Financial Assets [Abstract]
Cash and due from banks	$ 12,445
Interest-bearing deposits	67,394
Securities	370,242
Mortgages loans held for sale	3,015
Loans net	575,030
Accrued interest receivable	6,291
BOLI/Bank-owned life insurance	20,346
Financial Liabilities [Abstract]
Deposits	922,017
Accrued interest payable	178
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial Assets [Abstract]
Cash and due from banks	12,445
Interest-bearing deposits	67,394
Securities	0
Mortgages loans held for sale	0
Loans net	0
Accrued interest receivable	0
BOLI/Bank-owned life insurance	0
Financial Liabilities [Abstract]
Deposits	0
Accrued interest payable	0
Significant Other Observable Inputs (Level 2) [Member]
Financial Assets [Abstract]
Cash and due from banks	0
Interest-bearing deposits	0
Securities	368,554
Mortgages loans held for sale	3,015
Loans net	574,455
Accrued interest receivable	6,291
BOLI/Bank-owned life insurance	20,346
Financial Liabilities [Abstract]
Deposits	922,017
Accrued interest payable	178
Significant Unobservable Inputs (Level 3) [Member]
Financial Assets [Abstract]
Cash and due from banks	0
Interest-bearing deposits	0
Securities	0
Mortgages loans held for sale	0
Loans net	575
Accrued interest receivable	0
BOLI/Bank-owned life insurance	0
Financial Liabilities [Abstract]
Deposits	0
Accrued interest payable	0
Carrying Amount [Member]
Financial Assets [Abstract]
Cash and due from banks	12,445	11,897
Interest-bearing deposits	67,394	98,355
Securities	358,119	318,913
Mortgages loans held for sale	3,015	2,623
Loans net	583,207	580,402
Accrued interest receivable	6,291	6,304
BOLI/Bank-owned life insurance	20,346	19,812
Financial Liabilities [Abstract]
Deposits	924,383	919,333
Accrued interest payable	178	206
Total Estimated Fair Value [Member]
Financial Assets [Abstract]
Cash and due from banks		11,897
Interest-bearing deposits		98,355
Securities		326,347
Mortgages loans held for sale		2,623
Loans net		572,357
Accrued interest receivable		6,304
BOLI/Bank-owned life insurance		19,812
Financial Liabilities [Abstract]
Deposits		913,882
Accrued interest payable		$ 206

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

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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

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Namespace Prefix:	nksh_
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Balance Type:	debit
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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

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Balance Type:	debit
Period Type:	instant

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

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Namespace Prefix:	nksh_
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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

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Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Balance Type:	na
Period Type:	duration

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 portion of a 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 the amount that could be realized under a life insurance contract or contracts owned by the Company as of the date of the statement of financial position. Such Company-owned life insurance policies are 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



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

-Publisher FASB

-Name Emerging Issues Task Force (EITF)

-Number 88-5

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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

-Publisher FASB

-Name FASB Technical Bulletin (FTB)

-Number 85-4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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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. This item represents the aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits among others.

+ References

No definition available.

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Balance Type:	na
Period Type:	duration

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 loan receivable as presented on the statement of financial position which may include mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts, or any other loans which are due the Company as of the balance sheet date.

+ 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



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 114

-Paragraph 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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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 mortgage loans held-for-sale as presented on the statement of financial position which are due the Company as of the balance sheet date.

+ References

No definition available.

+ Details

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