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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Jun. 30, 2022 | Dec. 31, 2021
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 554,162 | $ 266,414
Temporarily impaired securities, less than 12 months unrealized loss | 48,867 | 4,922
Temporarily impaired securities, 12 months or more fair value | 106,949 | 51,999
Temporarily impaired securities, 12 months or more unrealized loss | 22,130 | 2,145
US Treasury Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 955 |
Temporarily impaired securities, less than 12 months unrealized loss | 35 |
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 | 262,561 | 201,650
Temporarily impaired securities, less than 12 months unrealized loss | 22,913 | 3,530
Temporarily impaired securities, 12 months or more fair value | 64,695 | 26,792
Temporarily impaired securities, 12 months or more unrealized loss | 12,228 | 1,180
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 120,556 | 50,659
Temporarily impaired securities, less than 12 months unrealized loss | 20,031 | 1,214
Temporarily impaired securities, 12 months or more fair value | 32,812 | 20,542
Temporarily impaired securities, 12 months or more unrealized loss | 9,310 | 793
Commercial Mortgage-Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 164,176 | 13,139
Temporarily impaired securities, less than 12 months unrealized loss | 5,302 | 141
Temporarily impaired securities, 12 months or more fair value | 9,442 | 4,665
Temporarily impaired securities, 12 months or more unrealized loss | 592 | 172
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 5,914 | 966
Temporarily impaired securities, less than 12 months unrealized loss | 586 | 37
Temporarily impaired securities, 12 months or more fair value | 0 | 0
Temporarily impaired securities, 12 months or more unrealized loss | $ 0 | $ 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
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |