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Note 5 - Securities (Details) - Temporarily Impaired Securities (USD $) In Thousands, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012
--- | --- | ---
Temporarily Impaired Securities: |  |
Less Than 12 Months - Fair Value | $ 228,176 | $ 53,709
Less Than 12 Months - Unrealized Loss | 19,901 | 838
12 Months or More - Fair Value | 482 | 615
12 Months or More - Unrealized Loss | 135 | 175
US Government Agencies [Member] |  |
Temporarily Impaired Securities: |  |
Less Than 12 Months - Fair Value | 164,695 | 44,351
Less Than 12 Months - Unrealized Loss | 13,032 | 622
12 Months or More - Fair Value | 0 | 0
12 Months or More - Unrealized Loss | 0 | 0
States and Political Subdivisions [Member] |  |
Temporarily Impaired Securities: |  |
Less Than 12 Months - Fair Value | 58,968 | 9,358
Less Than 12 Months - Unrealized Loss | 6,365 | 216
12 Months or More - Fair Value | 309 | 482
12 Months or More - Unrealized Loss | 4 | 3
Corporate Securities [Member] |  |
Temporarily Impaired Securities: |  |
Less Than 12 Months - Fair Value | 4,513 |
Less Than 12 Months - Unrealized Loss | 504 |
12 Months or More - Fair Value | 0 |
12 Months or More - Unrealized Loss | 0 |
Other Securities [Member] |  |
Temporarily Impaired Securities: |  |
Less Than 12 Months - Fair Value | 0 | 0
Less Than 12 Months - Unrealized Loss | 0 | 0
12 Months or More - Fair Value | 173 | 133
12 Months or More - Unrealized Loss | $ 131 | $ 172

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
Name: | nksh_TemporarilyImpairedSecuritiesAbstract0
Namespace Prefix: | nksh_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration