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Note 3 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) $ in Thousands | Dec. 31, 2016 | Dec. 31, 2015
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Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | $ 295,295 | $ 96,678
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 7,232 | 2,116
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 2,475 | 95,447
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 178 | 3,698
US Government Agencies Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 260,150 | 88,255
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 6,161 | 1,800
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 996 | 84,959
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 4 | 3,329
US States and Political Subdivisions Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 31,257 | 3,449
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 934 | 24
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 1,316 | 10,161
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 148 | 306
Corporate Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 3,888 | 4,974
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 137 | 292
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 0 | 200
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 0 | 1
Other Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 0 | 0
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 0 | 0
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 163 | 127
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | $ 26 | $ 62

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
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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
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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 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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