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Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Dec. 31, 2022 | Dec. 31, 2021
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 389,208 | $ 266,414
Temporarily impaired securities, less than 12 months unrealized loss | 39,109 | 4,922
Temporarily impaired securities, 12 months or more fair value | 259,070 | 51,999
Temporarily impaired securities, 12 months or more unrealized loss | 64,043 | 2,145
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 144,574 | 201,650
Temporarily impaired securities, less than 12 months unrealized loss | 12,699 | 3,530
Temporarily impaired securities, 12 months or more fair value | 190,950 | 26,792
Temporarily impaired securities, 12 months or more unrealized loss | 42,303 | 1,180
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 94,657 |
Temporarily impaired securities, less than 12 months unrealized loss | 18,373 |
Temporarily impaired securities, 12 months or more fair value | 52,134 |
Temporarily impaired securities, 12 months or more unrealized loss | 19,645 |
US Treasury Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 936 | 50,659
Temporarily impaired securities, less than 12 months unrealized loss | 56 | 1,214
Temporarily impaired securities, 12 months or more fair value | 0 | 20,542
Temporarily impaired securities, 12 months or more unrealized loss | 0 | 793
Commercial Mortgage-Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 144,198 | 13,139
Temporarily impaired securities, less than 12 months unrealized loss | 7,326 | 141
Temporarily impaired securities, 12 months or more fair value | 15,165 | 4,665
Temporarily impaired securities, 12 months or more unrealized loss | 1,913 | 172
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 4,843 | 966
Temporarily impaired securities, less than 12 months unrealized loss | 655 | 37
Temporarily impaired securities, 12 months or more fair value | 821 | 0
Temporarily impaired securities, 12 months or more unrealized loss | $ 182 | $ 0

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
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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: |