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Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Mar. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 79,750 | $ 389,208
Temporarily impaired securities, less than 12 months unrealized loss | 3,615 | 39,109
Temporarily impaired securities, 12 months or more fair value | 570,128 | 259,070
Temporarily impaired securities, 12 months or more unrealized loss | 84,606 | 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 | 947 | 0
Temporarily impaired securities, 12 months or more unrealized loss | 46 | 0
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 50,869 | 144,574
Temporarily impaired securities, less than 12 months unrealized loss | 2,032 | 12,699
Temporarily impaired securities, 12 months or more fair value | 283,074 | 190,950
Temporarily impaired securities, 12 months or more unrealized loss | 46,616 | 42,303
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 9,432 | 94,657
Temporarily impaired securities, less than 12 months unrealized loss | 1,214 | 18,373
Temporarily impaired securities, 12 months or more fair value | 139,371 | 52,134
Temporarily impaired securities, 12 months or more unrealized loss | 30,041 | 19,645
Commercial Mortgage-Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 18,576 | 144,198
Temporarily impaired securities, less than 12 months unrealized loss | 242 | 7,326
Temporarily impaired securities, 12 months or more fair value | 141,868 | 15,165
Temporarily impaired securities, 12 months or more unrealized loss | 7,269 | 1,913
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 873 | 4,843
Temporarily impaired securities, less than 12 months unrealized loss | 127 | 655
Temporarily impaired securities, 12 months or more fair value | 4,868 | 821
Temporarily impaired securities, 12 months or more unrealized loss | $ 634 | $ 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
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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
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Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
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X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |