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Note 5 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) | Mar. 31, 2016 | Dec. 31, 2015
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US Government Agencies Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | $ 82,951,000 | $ 88,255,000
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 363,000 | 1,800,000
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value | 25,632,000 | 84,959,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses | 347,000 | 3,329,000
US States and Political Subdivisions Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 2,285,000 | 3,449,000
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 2,000 | 24,000
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value | 5,380,000 | 10,161,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses | 210,000 | 306,000
Corporate Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 2,184,000 | 4,974,000
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 67,000 | 292,000
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value | 200,000 | 200,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses | 1,000 | $ 1,000
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 | $ 0
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value | 125,000 | 127,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses | 64,000 | 62,000
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 87,420,000 | 96,678,000
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 432,000 | 2,116,000
Continuous Unrealized Loss Position Twelve Months or Longer Fair Value | 31,337,000 | 95,447,000
Continuous Unrealized Loss Position 12 Months or Longer Aggregate Losses | $ 622,000 | $ 3,698,000

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
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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_
Data Type: | xbrli:monetaryItemType
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X
- Details
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X
- Details
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- Details
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Namespace Prefix: |
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X
- Details
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Data Type: | na
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