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Note 8 - Fair Value Measurements (Details) - Estimated Fair Values and Related Carrying Amounts of Financial Instruments (USD $) In Thousands, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012
--- | --- | ---
Fair Value, Inputs, Level 1 [Member] |  |
Financial Assets: |  |
Cash and due from banks | $ 13,474 | $ 14,783
Interest-bearing deposits | 63,731 | 96,597
Securities | 0 | 0
Restricted securities | 0 | 0
Mortgage loans held for sale | 0 | 0
Loans, net | 0 | 0
Accrued interest receivable | 6,291 | 6,247
Bank-owned life insurance | 20,858 | 20,523
Financial Liabilities: |  |
Deposits | 678,921 | 669,028
Accrued interest payable | 126 | 139
Fair Value, Inputs, Level 2 [Member] |  |
Financial Assets: |  |
Cash and due from banks | 0 | 0
Interest-bearing deposits | 0 | 0
Securities | 369,832 | 360,661
Restricted securities | 1,414 | 1,689
Mortgage loans held for sale | 1,950 | 2,796
Loans, net | 0 | 0
Accrued interest receivable | 0 | 0
Bank-owned life insurance | 0 | 0
Financial Liabilities: |  |
Deposits | 0 | 0
Accrued interest payable | 0 | 0
Fair Value, Inputs, Level 3 [Member] |  |
Financial Assets: |  |
Cash and due from banks | 0 | 0
Interest-bearing deposits | 0 | 0
Securities | 0 | 0
Restricted securities | 0 | 0
Mortgage loans held for sale | 0 | 0
Loans, net | 600,826 | 570,471
Accrued interest receivable | 0 | 0
Bank-owned life insurance | 0 | 0
Financial Liabilities: |  |
Deposits | 258,326 | 272,820
Accrued interest payable | 0 | 0
Reported Value Measurement [Member] |  |
Financial Assets: |  |
Cash and due from banks | 13,474 | 14,783
Interest-bearing deposits | 63,731 | 96,597
Securities | 372,863 | 350,354
Restricted securities | 1,414 | 1,689
Mortgage loans held for sale | 1,950 | 2,796
Loans, net | 580,109 | 583,813
Accrued interest receivable | 6,291 | 6,247
Bank-owned life insurance | 20,858 | 20,523
Financial Liabilities: |  |
Deposits | 938,521 | 946,766
Accrued interest payable | 126 | 139
Estimate of Fair Value Measurement [Member] |  |
Financial Assets: |  |
Cash and due from banks | 13,474 | 14,783
Interest-bearing deposits | 63,731 | 96,597
Securities | 369,832 | 360,661
Restricted securities | 1,414 | 1,689
Mortgage loans held for sale | 1,950 | 2,796
Loans, net | 600,826 | 570,471
Accrued interest receivable | 6,291 | 6,247
Bank-owned life insurance | 20,858 | 20,523
Financial Liabilities: |  |
Deposits | 937,247 | 941,848
Accrued interest payable | $ 126 | $ 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