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Note 3 - Securities (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Note 3 - Securities (Details) [Line Items] |  |
Number of Temporarily Impaired Securities | 205 nksh_NumberOfTemporarilyImpairedSecurities | 286 nksh_NumberOfTemporarilyImpairedSecurities
Available-for-sale and Held-to-maturity Securities, Continuous Unrealized Loss Position, Fair Value | $ 186,678 nksh_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue | $ 220,508 nksh_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Unrealized Loss on Securities | 5,557 us-gaap_UnrealizedLossOnSecurities | 30,761 us-gaap_UnrealizedLossOnSecurities
Available-for-sale Securities Pledged as Collateral | 157,951 us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral | 139,873 us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
Restricted Stock [Member] |  |
Note 3 - Securities (Details) [Line Items] |  |
Restricted Investments | 1,089 us-gaap_RestrictedInvestments / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember | 1,414 us-gaap_RestrictedInvestments / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Federal Home Loan Bank of Atlanta [Member] | National Bank of Blacksburg [Member] |  |
Note 3 - Securities (Details) [Line Items] |  |
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged |  | $ 437,847 us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1 / us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis = us-gaap_FederalHomeLoanBankOfAtlantaMember / dei_LegalEntityAxis = nksh_NationalBankOfBlacksburgMember

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- 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
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- Definition This item contains disclosure of the number of investment positions determined to be temporarily impaired. + References No definition available. + Details
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- Definition The fair value of available-for-sale securities that serve as collateral for borrowings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5086-111524 + Details
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- Definition Amount of the collateral pledged in support of federal home loan bank advances outstanding. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 470 -Section 50 -Paragraph 3 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823 + Details
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- Definition Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet. + References No definition available. + Details
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- 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
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