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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 191,502 | $ 69,967
Temporarily impaired securities, less than 12 months unrealized loss | 8,805 | 498
Temporarily impaired securities, 12 months or more fair value | 4,092 | 4,695
Temporarily impaired securities, 12 months or more unrealized loss | 197 | 5
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 89,509 | 28,798
Temporarily impaired securities, less than 12 months unrealized loss | 4,378 | 173
Temporarily impaired securities, 12 months or more fair value |  | 0
Temporarily impaired securities, 12 months or more unrealized loss |  | 0
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 89,706 | 32,353
Temporarily impaired securities, less than 12 months unrealized loss | 4,198 | 249
Temporarily impaired securities, 12 months or more fair value | 4,092 | 635
Temporarily impaired securities, 12 months or more unrealized loss | 197 | 3
Commercial Mortgage Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 12,287 | 8,816
Temporarily impaired securities, less than 12 months unrealized loss | 229 | 76
Temporarily impaired securities, 12 months or more fair value | 0 | 4,060
Temporarily impaired securities, 12 months or more unrealized loss | $ 0 | $ 2

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_CommercialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |