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Note 1 - General (Details Textual) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 USD ($)
Accounting Standards Update 2014-09 [Member] | Period Ended March 31, 2017 [Member] |
Prior Period Reclassification Adjustment | $ 654

X
- Definition The amount of a reclassification adjustment made to prior period financial statement amounts. + References No definition available. + Details
Name: | us-gaap_PriorPeriodReclassificationAdjustment
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201409Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_CreationDateAxis=nksh_PeriodEndedMarch312017Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |