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Note 4 - Allowance for Loan Losses, Nonperforming Assets and Impaired Loans (Details) - Loans Past Due 90 Days or More and Impaired Loans (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Loans Past Due 90 Days or More and Impaired Loans [Abstract] |  |  |  |  |  |
Loans past due 90 days or more and still accruing | $ 272 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing | [1] | $ 163 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing |  | $ 207 us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing | [1]
Ratio of loans past due 90 days or more and still accruing to loans, net of unearned income and deferred fees | 0.04%nksh_RatioOfLoansPastDue90DaysOrMoreAndStillAccruingToLoans |  | 0.03%nksh_RatioOfLoansPastDue90DaysOrMoreAndStillAccruingToLoans |  | 0.03%nksh_RatioOfLoansPastDue90DaysOrMoreAndStillAccruingToLoans |
Accruing restructured loans | 5,992 nksh_AccruingRestructuredLoans |  | 6,145 nksh_AccruingRestructuredLoans |  | 6,040 nksh_AccruingRestructuredLoans |
Impaired loans with no valuation allowance | 11,265 us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment | [2],[3] | 10,139 us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment |  | 7,615 us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment | [2],[3]
Impaired loans with a valuation allowance | 3,729 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment | [2],[3] | 2,463 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment |  | 7,506 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment | [2],[3]
Total impaired loans | 14,994 us-gaap_ImpairedFinancingReceivableRecordedInvestment | [2],[3] | 12,602 us-gaap_ImpairedFinancingReceivableRecordedInvestment |  | 15,121 us-gaap_ImpairedFinancingReceivableRecordedInvestment | [2],[3]
Valuation allowance | (236)us-gaap_ImpairedFinancingReceivableRelatedAllowance | [3] | (280)us-gaap_ImpairedFinancingReceivableRelatedAllowance |  | (282)us-gaap_ImpairedFinancingReceivableRelatedAllowance | [3]
Impaired loans, net of allowance | 14,758 nksh_ImpairedLoansNetOfAllowance |  | 12,322 nksh_ImpairedLoansNetOfAllowance |  | 14,839 nksh_ImpairedLoansNetOfAllowance |
Average recorded investment in impaired loans(1) | 15,192 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | [2],[4] | 13,075 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | [2],[4] | 16,311 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | [2]
Interest income recognized on impaired loans, after designation as impaired | 174 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod | [4] | 99 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod | [4] | 473 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod |
Amount of income recognized on a cash basis | $ 0 us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod |  | $ 0 us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod |  | $ 0 us-gaap_ImpairedFinancingReceivableInterestIncomeCashBasisMethod |
[1] | Only classes with past-due or nonaccrual loans are shown.
[2] | Recorded investment is net of charge-offs and interest paid while a loan is in nonaccrual status.
[3] | Only classes with impaired loans are shown
[4] | Only classes with impaired loans are shown.

X
- Definition Reflects the carrying amount of loans modified in a troubled debt restructuring that are accruing interest. + References No definition available. + Details
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Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses, net of allowance. + References No definition available. + Details
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Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
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- Definition Represents ratio of loans past due 90 days or more and still accruing to loans, net of unearned income and deferred fees. + References No definition available. + Details
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Namespace Prefix: | nksh_
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- Definition Recorded investment in financing receivables that are 90 days or more past due and still accruing. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5111-111524 + Details
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- Definition Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524 + Details
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Balance Type: | debit
Period Type: | duration

X
- Definition Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 + Details
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Balance Type: | credit
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- Definition Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 + Details
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X
- Definition Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524 + Details
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Balance Type: | debit
Period Type: | instant

X
- Definition Amount of allowance for credit losses related to recorded investment. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 + Details
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X
- Definition Amount of investment in impaired financing receivables for which there is no related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524 + Details
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- Definition Amount of investment in impaired financing receivables for which there is a related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 14A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524 + Details
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