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Note 5 - Securities (Details) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2015 USD ($) | Dec. 31, 2014 USD ($)
Note 5 - Securities (Details) [Line Items] |  |
Number of Temporarily Impaired Securities | 245 |
Available-for-sale and Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | $ 212,176 |
Unrealized Loss on Securities | 10,937 |
Continuous Unrealized Loss Position, Twelve Months Or Longer, Fair Value | 98,516 | $ 178,042
Continuous Unrealized Loss Position, 12 Months Or Longer, Aggregate Losses | $ 6,593 | $ 5,490
Contingent Convertible Preferred Stock [Member] |  |
Note 5 - Securities (Details) [Line Items] |  |
Number of Temporarily Impaired Securities | 111 |

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
- References No definition available. + Details
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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: |