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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 77,054,000 | $ 39,454,000
Temporarily impaired securities, less than 12 months unrealized loss | 509,000 | 762,000
Temporarily impaired securities, 12 months or more fair value | 72,369,000 | 281,626,000
Temporarily impaired securities, 12 months or more unrealized loss | 265,000 | 7,365,000
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 27,231,000 | 17,730,000
Temporarily impaired securities, less than 12 months unrealized loss | 211,000 | 216,000
Temporarily impaired securities, 12 months or more fair value | 71,778,000 | 259,992,000
Temporarily impaired securities, 12 months or more unrealized loss | 217,000 | 6,450,000
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  | 16,882,000
Temporarily impaired securities, less than 12 months unrealized loss |  | 352,000
Temporarily impaired securities, 12 months or more fair value | 591,000 | 20,758,000
Temporarily impaired securities, 12 months or more unrealized loss | 48,000 | 814,000
Commercial Mortgage Backed Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 49,823,000 |
Temporarily impaired securities, less than 12 months unrealized loss | 298,000 |
Temporarily impaired securities, 12 months or more fair value |  |
Temporarily impaired securities, 12 months or more unrealized loss |  |
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  | 4,842,000
Temporarily impaired securities, less than 12 months unrealized loss |  | 194,000
Temporarily impaired securities, 12 months or more fair value |  | 876,000
Temporarily impaired securities, 12 months or more unrealized loss |  | $ 101,000

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: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |