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Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Temporarily impaired securities, less than 12 months, fair value | $ 87,068 | $ 295,295
Temporarily impaired securities, less than 12 months unrealized loss | 1,065 | 7,232
Temporarily impaired securities, 12 months or more fair value | 229,675 | 2,475
Temporarily impaired securities, 12 months or more unrealized loss | 4,750 | 178
US Government Agencies Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 68,380 | 260,150
Temporarily impaired securities, less than 12 months unrealized loss | 871 | 6,161
Temporarily impaired securities, 12 months or more fair value | 225,738 | 996
Temporarily impaired securities, 12 months or more unrealized loss | 4,623 | 4
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value | 18,688 | 31,257
Temporarily impaired securities, less than 12 months unrealized loss | 194 | 934
Temporarily impaired securities, 12 months or more fair value | 2,989 | 1,316
Temporarily impaired securities, 12 months or more unrealized loss | 98 | 148
Corporate Debt Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  | 3,888
Temporarily impaired securities, less than 12 months unrealized loss |  | 137
Temporarily impaired securities, 12 months or more fair value | 948 | 0
Temporarily impaired securities, 12 months or more unrealized loss | $ 29 | 0
Other Securities [Member] |  |
Temporarily impaired securities, less than 12 months, fair value |  | 0
Temporarily impaired securities, less than 12 months unrealized loss |  | 0
Temporarily impaired securities, 12 months or more fair value |  | 163
Temporarily impaired securities, 12 months or more unrealized loss |  | $ 26

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
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Namespace Prefix: | nksh_
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Balance Type: | credit
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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
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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_
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X
- Details
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- Details
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- Details
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X
- Details
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Data Type: | na
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Period Type: |