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Note 5 - Securities (Details Textual) | Sep. 30, 2016 USD ($) | Dec. 31, 2015 USD ($)
--- | --- | ---
Restricted Stock [Member] |  |
Restricted Investments | $ 1,170,000 | $ 1,129,000
US States and Political Subdivisions Debt Securities [Member] |  |
Number of Temporarily Impaired Securities | 2 |
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value | $ 1,440,000 |
Continuous Unrealized Loss Position Aggregate Losses | 24,000 |
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value | 1,440,000 | 10,161,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses | $ 24,000 | 306,000
Other Securities [Member] |  |
Number of Temporarily Impaired Securities | 1 |
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value | $ 152,000 |
Continuous Unrealized Loss Position Aggregate Losses | 37,000 |
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value | 152,000 | 127,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses | $ 37,000 | 62,000
Number of Temporarily Impaired Securities | 95 |
Available-for-sale and Held to Maturity Securities Continuous Unrealized Loss Position Fair Value | $ 93,095,000 |
Continuous Unrealized Loss Position Aggregate Losses | $ 315,000 |
Number of Temporarily Impaired Securities Greater than Twelve Months | 3 |
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value | $ 1,592,000 | 95,447,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses | $ 61,000 | $ 3,698,000

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 Amount of the excess of amortized cost basis over fair value of securities that are in a loss position. + References No definition available. + Details
Name: | nksh_ContinuousUnrealizedLossPositionAggregateLosses
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
- 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 This item contains disclosure of the number of investment positions determined to be temporarily impaired in a continuous loss position for twelve months or more. + References No definition available. + Details
Name: | nksh_NumberOfTemporarilyImpairedSecuritiesGreaterThanTwelveMonths
Namespace Prefix: | nksh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet. + References No definition available. + Details
Name: | us-gaap_RestrictedInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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