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Restatement of Consolidated Financial Statements, Assets Fair Value Measurements And Inputs (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |  |  |  |  |  |  |  |  |
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Dec. 31, 2012 | Dec. 31, 2012 As Reported [Member] Impaired loans [Member] | Dec. 31, 2012 As Restated [Member] Maximum [Member] Impaired loans [Member] | Dec. 31, 2012 As Restated [Member] Minimum [Member] Impaired loans [Member] | Dec. 31, 2012 As Restated [Member] Weighted Average [Member] Impaired loans [Member] | Dec. 31, 2012 Nonrecurring [Member] | Dec. 31, 2011 Nonrecurring [Member] | Dec. 31, 2012 Nonrecurring [Member] As Reported [Member] | Dec. 31, 2012 Nonrecurring [Member] As Restated [Member] | Dec. 31, 2012 Nonrecurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Dec. 31, 2011 Nonrecurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Dec. 31, 2012 Nonrecurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] As Reported [Member] | Dec. 31, 2012 Nonrecurring [Member] Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] As Restated [Member] | Dec. 31, 2012 Nonrecurring [Member] Significant Other Observable Inputs (Level 2) [Member] | Dec. 31, 2011 Nonrecurring [Member] Significant Other Observable Inputs (Level 2) [Member] | Dec. 31, 2012 Nonrecurring [Member] Significant Other Observable Inputs (Level 2) [Member] As Reported [Member] | Dec. 31, 2012 Nonrecurring [Member] Significant Other Observable Inputs (Level 2) [Member] As Restated [Member] | Dec. 31, 2012 Nonrecurring [Member] Significant Unobservable Inputs (Level 3) [Member] | Dec. 31, 2011 Nonrecurring [Member] Significant Unobservable Inputs (Level 3) [Member] | Dec. 31, 2012 Nonrecurring [Member] Significant Unobservable Inputs (Level 3) [Member] As Reported [Member] | Dec. 31, 2012 Nonrecurring [Member] Significant Unobservable Inputs (Level 3) [Member] As Restated [Member]
Fair Value Assets Measured on Nonrecurring Basis and fair value inputs [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Impaired Loans Net Fair Value Disclosure |  |  |  |  |  | $ 928 | $ 5,968 | $ 362 | $ 928 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | $ 928 | $ 5,968 | $ 362 | $ 928
Fair value inputs [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Valuation Technique | Present value of cash flows |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount Rate |  | 6.00% | 7.50% | 6.00% | 6.28% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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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 element represents the fair value of impaired loans net of valuation allowance. + References No definition available. + Details
Name: | nksh_ImpairedLoansNetFairValueDisclosure
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | nksh_RestatedFairValueAssetsAndLiabilitiesMeasuredOnNonrecurringBasisLineItems
Namespace Prefix: | nksh_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FairValueInputsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueInputsDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Description of the inputs and valuation technique(s) used to measure fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258 + Details
Name: | us-gaap_FairValueMeasurementsValuationTechniques
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration