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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Sep. 30, 2022 | Dec. 31, 2021
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 465,460 | $ 266,414
Temporarily impaired securities, less than 12 months unrealized loss | 57,475 | 4,922
Temporarily impaired securities, 12 months or more fair value | 182,187 | 51,999
Temporarily impaired securities, 12 months or more unrealized loss | 50,582 | 2,145
US Treasury Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 932 |
Temporarily impaired securities, less than 12 months unrealized loss | 59 |
Temporarily impaired securities, 12 months or more fair value | 0 |
Temporarily impaired securities, 12 months or more unrealized loss | 0 |
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 207,506 | 201,650
Temporarily impaired securities, less than 12 months unrealized loss | 25,296 | 3,530
Temporarily impaired securities, 12 months or more fair value | 124,968 | 26,792
Temporarily impaired securities, 12 months or more unrealized loss | 31,692 | 1,180
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 100,072 | 50,659
Temporarily impaired securities, less than 12 months unrealized loss | 23,747 | 1,214
Temporarily impaired securities, 12 months or more fair value | 43,622 | 20,542
Temporarily impaired securities, 12 months or more unrealized loss | 17,482 | 793
Commercial Mortgage-Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 152,201 | 13,139
Temporarily impaired securities, less than 12 months unrealized loss | 7,624 | 141
Temporarily impaired securities, 12 months or more fair value | 12,802 | 4,665
Temporarily impaired securities, 12 months or more unrealized loss | 1,200 | 172
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 4,749 | 966
Temporarily impaired securities, less than 12 months unrealized loss | 749 | 37
Temporarily impaired securities, 12 months or more fair value | 795 | 0
Temporarily impaired securities, 12 months or more unrealized loss | $ 208 | $ 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
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
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |