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Note 3 - Securities - Realized Gains and Losses From Calls of Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020
Available for sale, proceeds | $ 20,377 | $ 126,840
Available for sale, book value | 20,371 | 126,732
Available for sale, gross gain | 6 | 110
Available for sale, gross loss | 0 | 2
Available for sale, net gain (loss) | $ 6 | $ 108

X
- Definition Book value of available-for-sale securities that were called during the period. + References No definition available. + Details
Name: | nksh_AvailableforsaleSecuritiesCalled
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The gross realized gain on available-for-sale securities that were called during the period. + References No definition available. + Details
Name: | nksh_AvailableforsaleSecuritiesGrossRealizedGainCalled
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The gross realized gain (loss) on available-for-sale securities that were called during the period. + References No definition available. + Details
Name: | nksh_AvailableforsaleSecuritiesGrossRealizedGainLossCalled
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The gross realized loss on available-for-sale securities that were called during the period. + References No definition available. + Details
Name: | nksh_AvailableforsaleSecuritiesGrossRealizedLossCalled
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The cash inflow associated with calls (requests of early payments) on securities not classified as either held-to-maturity securities or trading securities which are classified as available-for-sale securities. + References No definition available. + Details
Name: | nksh_ProceedsFromCallsOfAvailableforsaleSecurities
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration