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Note 5 - Securities - Securities in a Continuous Loss Position (Details) - USD ($) | Sep. 30, 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 | $ 90,723,000 | $ 88,255,000
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 253,000 | 1,800,000
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 0 | 84,959,000
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 0 | 3,329,000
US States and Political Subdivisions Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 780,000 | 3,449,000
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 1,000 | 24,000
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 1,440,000 | 10,161,000
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 24,000 | 306,000
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 | 152,000 | 127,000
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | 37,000 | 62,000
Corporate Debt Securities [Member] |  |
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value |  | 4,974,000
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss |  | 292,000
Securities in a Continuours Loss Position, 12 Months or More, Fair Value |  | 200,000
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss |  | 1,000
Securities in a Continuours Loss Position, Less Than 12 Months, Fair Value | 91,503,000 | 96,678,000
Securities in a Continuours Loss Position, Less Than 12 Months, Unrealized Loss | 254,000 | 2,116,000
Securities in a Continuours Loss Position, 12 Months or More, Fair Value | 1,592,000 | 95,447,000
Securities in a Continuours Loss Position, 12 Months or More, Unrealized Loss | $ 61,000 | $ 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
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
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=nksh_OtherSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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