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Note 5 - Securities (Details) - Securities in a Continuous Loss Position (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Temporarily Impaired Securities: |  |
Securities in a continuous loss position, less than 12 months, fair value | $ 36,106 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue | $ 8,636 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Securities in a continuous loss position, less than 12 months, unrealized loss | 272 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses | 67 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses
Securities in a continuous loss position, 12 months or more, fair value | 105,032 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue | 178,042 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Securities in a continuous loss position, 12 months or more, unrealized loss | 1,826 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 5,490 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
US Government Agencies Debt Securities [Member] |  |
Temporarily Impaired Securities: |  |
Securities in a continuous loss position, less than 12 months, fair value | 25,832 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 6,964 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Securities in a continuous loss position, less than 12 months, unrealized loss | 163 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 30 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Securities in a continuous loss position, 12 months or more, fair value | 94,959 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 156,149 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Securities in a continuous loss position, 12 months or more, unrealized loss | 1,324 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 4,709 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
US States and Political Subdivisions Debt Securities [Member] |  |
Temporarily Impaired Securities: |  |
Securities in a continuous loss position, less than 12 months, fair value | 10,274 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 1,222 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities in a continuous loss position, less than 12 months, unrealized loss | 109 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 35 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities in a continuous loss position, 12 months or more, fair value | 9,952 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 19,818 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities in a continuous loss position, 12 months or more, unrealized loss | 434 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 692 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Debt Securities [Member] |  |
Temporarily Impaired Securities: |  |
Securities in a continuous loss position, less than 12 months, fair value |  | 450 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Securities in a continuous loss position, less than 12 months, unrealized loss |  | 2 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Securities in a continuous loss position, 12 months or more, fair value |  | 1,948 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Securities in a continuous loss position, 12 months or more, unrealized loss |  | 27 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Other Debt Obligations [Member] |  |
Temporarily Impaired Securities: |  |
Securities in a continuous loss position, 12 months or more, fair value | 121 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember | 127 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember
Securities in a continuous loss position, 12 months or more, unrealized loss | $ 68 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember | $ 62 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember

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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Balance Type: | credit
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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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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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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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