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Note 4 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016
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Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | $ 289,043 | $ 295,295
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 5,255 | 7,232
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 3,564 | 2,475
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 88 | 178
US Government Agencies Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 264,648 | 260,150
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 4,645 | 6,161
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 1,995 | 996
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 4 | 4
US States and Political Subdivisions Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 21,505 | 31,257
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 483 | 934
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 1,389 | 1,316
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 75 | 148
Corporate Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 2,890 | 3,888
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 127 | 137
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 0 |
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 0 | 0
Other Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 0 |
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 0 |
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 180 | 163
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | $ 9 | $ 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_
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
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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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X
- Details
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Data Type: | na
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