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Fair Value Measurements (Details) (Recurring [Member], USD $) In Thousands, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Financial assets and liabilities measured at fair value on a recurring basis [Abstract] |  |
U.S. Treasury | $ 2,053 | $ 2,073
U.S. Government agencies and corporations | 137,638 | 129,564
State and political subdivisions | 33,583 | 36,779
Mortgage-backed Securities | 4,072 | 4,569
Corporate debt securities | 11,935 | 14,575
Other securities | 2,259 | 2,255
Total securities available for sale | 191,540 | 189,815
Assets [Abstract] |  |
Impaired loans net of valuation allowance | 722 | 928
Other real estate owned net of valuation allowance | 1,094 | 1,435
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] |  |
Financial assets and liabilities measured at fair value on a recurring basis [Abstract] |  |
U.S. Treasury | 0 | 0
U.S. Government agencies and corporations | 0 | 0
State and political subdivisions | 0 | 0
Mortgage-backed Securities | 0 | 0
Corporate debt securities | 0 | 0
Other securities | 0 | 0
Total securities available for sale | 0 | 0
Assets [Abstract] |  |
Impaired loans net of valuation allowance | 0 | 0
Other real estate owned net of valuation allowance | 0 | 0
Significant Other Observable Inputs (Level 2) [Member] |  |
Financial assets and liabilities measured at fair value on a recurring basis [Abstract] |  |
U.S. Treasury | 2,053 | 2,073
U.S. Government agencies and corporations | 137,638 | 129,564
State and political subdivisions | 33,583 | 36,779
Mortgage-backed Securities | 4,072 | 4,569
Corporate debt securities | 11,935 | 14,575
Other securities | 2,259 | 2,255
Total securities available for sale | 191,540 | 189,815
Assets [Abstract] |  |
Impaired loans net of valuation allowance | 0 | 0
Other real estate owned net of valuation allowance | 0 | 0
Significant Unobservable Inputs (Level 3) [Member] |  |
Financial assets and liabilities measured at fair value on a recurring basis [Abstract] |  |
U.S. Treasury | 0 | 0
U.S. Government agencies and corporations | 0 | 0
State and political subdivisions | 0 | 0
Mortgage-backed Securities | 0 | 0
Corporate debt securities | 0 | 0
Other securities | 0 | 0
Total securities available for sale | 0 | 0
Assets [Abstract] |  |
Impaired loans net of valuation allowance | 722 | 928
Other real estate owned net of valuation allowance | $ 1,094 | $ 1,435

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 debt securities issued by corporations. + References No definition available. + Details
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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 element represents the fair value of impaired loans net of valuation allowance. + References No definition available. + Details
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Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
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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 element represents the fair value of other real estate owned by bank. + References No definition available. + Details
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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 debt issued by US states and political subdivisions. + References No definition available. + Details
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Namespace Prefix: | nksh_
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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 US government agency issued debt securities. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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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 US government issued debt securities. + References No definition available. + Details
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- Details
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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 Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph b -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 Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 + Details
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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 securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages) as of the balance sheet date which have been categorized as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-DEP -Paragraph 15 -IssueDate 2006-05-01 -Chapter 7 -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Mortgage-Backed Securities -URI http://asc.fasb.org/extlink&oid=6517959 + Details
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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 element represents the fair value of assets categorized as other which are not in and of themselves material enough to require separate disclosure. + References No definition available. + Details
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