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Note 3 - Securities (Details) - Securities in a Continuous Loss Position (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Note 3 - Securities (Details) - Securities in a Continuous Loss Position [Line Items] |  |
Securities in a continuous loss position, less than 12 months, fair value | $ 8,636 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue | $ 202,015 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Securities in a continuous loss position, less than 12 months, unrealized loss | 67 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses | 27,059 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses
Securities in a continuous loss position, 12 months or more, fair value | 178,042 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue | 18,493 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Securities in a continuous loss position, 12 months or more, unrealized loss | 5,490 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses | 3,702 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
US Government Agencies Debt Securities [Member] |  |
Note 3 - Securities (Details) - Securities in a Continuous Loss Position [Line Items] |  |
Securities in a continuous loss position, less than 12 months, fair value | 6,964 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 138,324 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Securities in a continuous loss position, less than 12 months, unrealized loss | 30 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 20,400 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Securities in a continuous loss position, 12 months or more, fair value | 156,149 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 15,796 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Securities in a continuous loss position, 12 months or more, unrealized loss | 4,709 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember | 3,211 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentAgenciesDebtSecuritiesMember
US States and Political Subdivisions Debt Securities [Member] |  |
Note 3 - Securities (Details) - Securities in a Continuous Loss Position [Line Items] |  |
Securities in a continuous loss position, less than 12 months, fair value | 1,222 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 58,013 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities in a continuous loss position, less than 12 months, unrealized loss | 35 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 6,131 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities in a continuous loss position, 12 months or more, fair value | 19,818 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 2,697 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Securities in a continuous loss position, 12 months or more, unrealized loss | 692 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 491 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Debt Securities [Member] |  |
Note 3 - Securities (Details) - Securities in a Continuous Loss Position [Line Items] |  |
Securities in a continuous loss position, less than 12 months, fair value | 450 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember | 5,511 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 | 506 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] |  |
Note 3 - Securities (Details) - Securities in a Continuous Loss Position [Line Items] |  |
Securities in a continuous loss position, less than 12 months, fair value |  | 167 nksh_ContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember
Securities in a continuous loss position, less than 12 months, unrealized loss |  | 22 nksh_ContinuousUnrealizedLossPositionLessThan12MonthsAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember
Securities in a continuous loss position, 12 months or more, fair value | 127 nksh_ContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember |
Securities in a continuous loss position, 12 months or more, unrealized loss | $ 62 nksh_ContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember |

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- 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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- 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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- 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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- 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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