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Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 31,829 | $ 389,208
Temporarily impaired securities, less than 12 months unrealized loss | 1,232 | 39,109
Temporarily impaired securities, 12 months or more fair value | 582,007 | 259,070
Temporarily impaired securities, 12 months or more unrealized loss | 89,779 | 64,043
US Treasury Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 0 | 936
Temporarily impaired securities, less than 12 months unrealized loss | 0 | 56
Temporarily impaired securities, 12 months or more fair value | 940 | 0
Temporarily impaired securities, 12 months or more unrealized loss | 54 | 0
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 24,782 | 144,574
Temporarily impaired securities, less than 12 months unrealized loss | 1,131 | 12,699
Temporarily impaired securities, 12 months or more fair value | 280,119 | 190,950
Temporarily impaired securities, 12 months or more unrealized loss | 47,756 | 42,303
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 2,070 | 94,657
Temporarily impaired securities, less than 12 months unrealized loss | 35 | 18,373
Temporarily impaired securities, 12 months or more fair value | 144,831 | 52,134
Temporarily impaired securities, 12 months or more unrealized loss | 32,708 | 19,645
Commercial Mortgage-Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 4,977 | 144,198
Temporarily impaired securities, less than 12 months unrealized loss | 66 | 7,326
Temporarily impaired securities, 12 months or more fair value | 150,514 | 15,165
Temporarily impaired securities, 12 months or more unrealized loss | 8,361 | 1,913
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 0 | 4,843
Temporarily impaired securities, less than 12 months unrealized loss | 0 | 655
Temporarily impaired securities, 12 months or more fair value | 5,603 | 821
Temporarily impaired securities, 12 months or more unrealized loss | $ 900 | $ 182

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

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: |

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