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Fair Value Measurements, Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Carrying Amount [Member] |  |
Financial assets [Abstract] |  |
Cash and due from banks | $ 10,674 | $ 14,783
Interest-bearing deposits | 107,176 | 96,597
Securities | 358,487 | 352,043
Mortgages loans held for sale | 313 | 2,796
Loans net | 566,667 | 583,813
Accrued interest receivable | 5,878 | 6,247
Bank-owned life insurance | 20,690 | 20,523
Financial liabilities [Abstract] |  |
Deposits | 937,119 | 946,766
Accrued interest payable | 146 | 139
Estimated Fair Value [Member] |  |
Financial assets [Abstract] |  |
Cash and due from banks | 10,674 | 14,783
Interest-bearing deposits | 107,176 | 96,597
Securities | 365,758 | 362,350
Mortgages loans held for sale | 313 | 2,796
Loans net | 555,322 | 570,471
Accrued interest receivable | 5,878 | 6,247
Bank-owned life insurance | 20,690 | 20,523
Financial liabilities [Abstract] |  |
Deposits | 931,041 | 941,848
Accrued interest payable | 146 | 139
Level 1 [Member] |  |
Financial assets [Abstract] |  |
Cash and due from banks | 10,674 | 14,783
Interest-bearing deposits | 107,176 | 96,597
Securities | 0 | 0
Mortgages loans held for sale | 0 | 0
Loans net | 0 | 0
Accrued interest receivable | 5,878 | 6,247
Bank-owned life insurance | 20,690 | 20,523
Financial liabilities [Abstract] |  |
Deposits | 667,227 | 669,028
Accrued interest payable | 146 | 139
Level 2 [Member] |  |
Financial assets [Abstract] |  |
Cash and due from banks | 0 | 0
Interest-bearing deposits | 0 | 0
Securities | 365,758 | 362,350
Mortgages loans held for sale | 313 | 2,796
Loans net | 0 | 0
Accrued interest receivable | 0 | 0
Bank-owned life insurance | 0 | 0
Financial liabilities [Abstract] |  |
Deposits | 0 | 0
Accrued interest payable | 0 | 0
Level 3 [Member] |  |
Financial assets [Abstract] |  |
Cash and due from banks | 0 | 0
Interest-bearing deposits | 0 | 0
Securities | 0 | 0
Mortgages loans held for sale | 0 | 0
Loans net | 555,322 | 570,471
Accrued interest receivable | 0 | 0
Bank-owned life insurance | 0 | 0
Financial liabilities [Abstract] |  |
Deposits | 263,814 | 272,820
Accrued interest payable | $ 0 | $ 0

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_
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
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Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
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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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 securities which consist of all investments in certain debt and equity securities. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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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 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. + 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. 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. + Details
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- 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. + Details
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- 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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