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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 182,732 | $ 270,457
Temporarily impaired securities, less than 12 months unrealized loss | 4,485 | 2,769
Temporarily impaired securities, 12 months or more fair value | 596 | 39,553
Temporarily impaired securities, 12 months or more unrealized loss | 42 | 85
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 4,972 | 53,244
Temporarily impaired securities, less than 12 months unrealized loss | 24 | 738
Temporarily impaired securities, 12 months or more fair value |  | 38,962
Temporarily impaired securities, 12 months or more unrealized loss |  | 37
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 14,157 | 35,934
Temporarily impaired securities, less than 12 months unrealized loss | 500 | 596
Temporarily impaired securities, 12 months or more fair value | 596 | 591
Temporarily impaired securities, 12 months or more unrealized loss | 42 | 48
Commercial Mortgage Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 163,603 | 181,279
Temporarily impaired securities, less than 12 months unrealized loss | 3,961 | 1,435
Temporarily impaired securities, 12 months or more fair value |  |
Temporarily impaired securities, 12 months or more unrealized loss |  |

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
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: |