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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 115,489 | $ 87,068
Temporarily impaired securities, less than 12 months unrealized loss | 4,027 | 1,065
Temporarily impaired securities, 12 months or more fair value | 250,876 | 229,675
Temporarily impaired securities, 12 months or more unrealized loss | 7,693 | 4,750
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 68,197 | 68,380
Temporarily impaired securities, less than 12 months unrealized loss | 2,570 | 871
Temporarily impaired securities, 12 months or more fair value | 244,485 | 225,738
Temporarily impaired securities, 12 months or more unrealized loss | 7,181 | 4,623
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 42,422 | 18,688
Temporarily impaired securities, less than 12 months unrealized loss | 1,293 | 194
Temporarily impaired securities, 12 months or more fair value | 5,514 | 2,989
Temporarily impaired securities, 12 months or more unrealized loss | 411 | 98
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 4,870 |
Temporarily impaired securities, less than 12 months unrealized loss | 164 |
Temporarily impaired securities, 12 months or more fair value | 877 | 948
Temporarily impaired securities, 12 months or more unrealized loss | $ 101 | $ 29

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 have been in a loss position for less than twelve months for securities. + References No definition available. + Details
Name: | nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses
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 less than twelve months. + References No definition available. + Details
Name: | nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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: | us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |