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Note 15 - Concentrations of Credit Risk (Details) (Credit Concentration Risk [Member], USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Commercial Real Estate [Member] |  |
Note 15 - Concentrations of Credit Risk (Details) [Line Items] |  |
Concentration Risk, Percentage | 52.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_CommercialRealEstateLoansMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember | 52.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_CommercialRealEstateLoansMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure | $ 310,762 us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_CommercialRealEstateLoansMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember | $ 311,266,000 us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_CommercialRealEstateLoansMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember
College Housing and Professional Office Buildings [Member] |  |
Note 15 - Concentrations of Credit Risk (Details) [Line Items] |  |
Concentration Risk, Percentage | 31.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_CollegeHousingAndProfessionalOfficeBuildingsMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember | 32.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_CollegeHousingAndProfessionalOfficeBuildingsMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure | 190,055,000 us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_CollegeHousingAndProfessionalOfficeBuildingsMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember | 190,866,000 us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_CollegeHousingAndProfessionalOfficeBuildingsMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember
Residential [Member] |  |
Note 15 - Concentrations of Credit Risk (Details) [Line Items] |  |
Concentration Risk, Percentage | 24.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_ResidentialMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember | 24.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_ResidentialMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure | 147,039,000 us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_ResidentialMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember | 145,499,000 us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_ResidentialMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember
Automobile [Member] |  |
Note 15 - Concentrations of Credit Risk (Details) [Line Items] |  |
Concentration Risk, Percentage | 2.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_AutomobileMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember | 2.00%us-gaap_ConcentrationRiskPercentage1 / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_AutomobileMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure | $ 11,554 us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_AutomobileMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember | $ 11,580,000 us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_ConcentrationRiskByBenchmarkAxis = nksh_AutomobileMember / us-gaap_ConcentrationRiskByTypeAxis = us-gaap_CreditConcentrationRiskMember

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- Definition Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611 + Details
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- Definition For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 275 -SubTopic 10 -Section 50 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592 + Details
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