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Note 5 - Securities (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014
Note 5 - Securities (Details) [Line Items] |  |
Number of Temporarily Impaired Securities | 154 nksh_NumberOfTemporarilyImpairedSecurities |
Available-for-sale and Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | $ 141,138 nksh_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue |
Unrealized Loss on Securities | 2,098 us-gaap_UnrealizedLossOnSecurities |
Continuous Unrealized Loss Position, Twelve Months Or Longer, Fair Value | 105,032 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue | 178,042 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Continuous Unrealized Loss Position, 12 Months Or Longer, Aggregate Losses | $ 1,826 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | $ 5,490 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
Contingent Convertible Preferred Stock [Member] |  |
Note 5 - Securities (Details) [Line Items] |  |
Number of Temporarily Impaired Securities | 112 nksh_NumberOfTemporarilyImpairedSecurities / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_ContingentConvertiblePreferredStockMember |

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized as held-to-maturity and available for sale securities that have been in a continuous unrealized loss position. + References No definition available. + Details
Name: | nksh_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities. + References No definition available. + Details
Name: | nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities that have been in a continuous unrealized loss position for twelve months or longer. + References No definition available. + Details
Name: | nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | nksh_Note5SecuritiesDetailsLineItems
Namespace Prefix: | nksh_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition This item contains disclosure of the number of investment positions determined to be temporarily impaired. + References No definition available. + Details
Name: | nksh_NumberOfTemporarilyImpairedSecurities
Namespace Prefix: | nksh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition The increase (decrease), resulting in a loss, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period. + References No definition available. + Details
Name: | us-gaap_UnrealizedLossOnSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ContingentConvertiblePreferredStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |