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Consolidated Statements of Income (Unaudited) (Parenthetical) (USD $) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Interest Expense |  |  |  |
Threshold limit of time deposits, minimum | $ 100,000 | $ 100,000 | $ 100,000 | $ 100,000

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- Definition Represents minimum threshold limit of time deposit for which interest to be recognized separately. + References No definition available. + Details
Name: | nksh_MinimumThresholdLimitOfTimeDeposits
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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