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Securities, Temporarily Impaired Securities (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2013 Security | Mar. 31, 2012 Security | Dec. 31, 2012
Temporarily impaired securities with gross unrealized losses and fair value [Abstract] |  |  |
Less Than 12 Months, Fair Value | $ 117,252 |  | $ 53,709
Less Than 12 Months, Unrealized Loss | 2,401 |  | 838
12 Months or More, Fair Value | 841 |  | 615
12 Months or More, Unrealized Loss | 138 |  | 175
Number of temporarily impaired securities | 137 | 4 |
Fair Value of temporarily impaired securities | 118,093 | 841 |
Unrealized loss on securities | 2,539 | 137 |
U.S. Government Agencies and Corporations [Member] |  |  |
Temporarily impaired securities with gross unrealized losses and fair value [Abstract] |  |  |
Less Than 12 Months, Fair Value | 85,100 |  | 44,351
Less Than 12 Months, Unrealized Loss | 1,222 |  | 622
12 Months or More, Fair Value | 0 |  | 0
12 Months or More, Unrealized Loss | 0 |  | 0
State and Political Subdivisions [Member] |  |  |
Temporarily impaired securities with gross unrealized losses and fair value [Abstract] |  |  |
Less Than 12 Months, Fair Value | 28,178 |  | 9,358
Less Than 12 Months, Unrealized Loss | 1,127 |  | 216
12 Months or More, Fair Value | 481 |  | 482
12 Months or More, Unrealized Loss | 4 |  | 3
Corporate Debt Securities [Member] |  |  |
Temporarily impaired securities with gross unrealized losses and fair value [Abstract] |  |  |
Less Than 12 Months, Fair Value | 3,974 |  |
Less Than 12 Months, Unrealized Loss | 52 |  |
12 Months or More, Fair Value | 0 |  |
12 Months or More, Unrealized Loss | 0 |  |
Other [Member] |  |  |
Temporarily impaired securities with gross unrealized losses and fair value [Abstract] |  |  |
Less Than 12 Months, Fair Value | 0 |  | 0
Less Than 12 Months, Unrealized Loss | 0 |  | 0
12 Months or More, Fair Value | 360 |  | 133
12 Months or More, Unrealized Loss | $ 134 |  | $ 172

X
- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized as held-to-maturity and available for sale securities that have been in a continuous unrealized loss position. + References No definition available. + Details
Name: | nksh_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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 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
- Definition This item contains disclosure of the number of investment positions determined to be temporarily impaired. + References No definition available. + Details
Name: | nksh_NumberOfTemporarilyImpairedSecurities
Namespace Prefix: | nksh_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | nksh_TemporarilyImpairedSecuritiesWithGrossUnrealizedLossesAndFairValueAbstract
Namespace Prefix: | nksh_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The increase (decrease), resulting in a loss, in the difference between the fair value and the carrying value, or in the comparative fair values, of securities held at each balance sheet date, that was included in earnings for the period. + References No definition available. + Details
Name: | us-gaap_UnrealizedLossOnSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration