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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Available for sale, proceeds | $ 13,620 | $ 220,520 | $ 63,775
Available for sale, book value | 13,614 | 220,334 | 63,742
Available for sale, gross gain | 6 | 186 | 34
Available for sale, gross loss | 0 |  | 1
Available for sale, net gain (loss) | 6 | 186 | 33
Held to maturity, proceeds | 8,975 | 16,160 | 7,605
Held to maturity, book value | 8,971 | 16,114 | 7,605
Held to maturity, gross gain | 4 | 46 |
Held to maturity, gross loss | 0 | 0 |
Held to maturity, net gain (loss) | $ 4 | $ 46 | $ 0

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 Book value of held-to-maturity securities that were called during the period. + References No definition available. + Details
Name: | nksh_HeldtomaturitySecuritiesCalled
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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

X
- Definition The gross realized loss on held-to-maturity securities that were called during the period. + References No definition available. + Details
Name: | nksh_HeldtomaturitySecuritiesGrossRealizedLossCalled
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

X
- Definition The cash inflow associated with the calls (requests for early payments) of debt securities designated as held-to-maturity. + References No definition available. + Details
Name: | nksh_ProceedsFromCallsOfHeldtomaturitySecurities
Namespace Prefix: | nksh_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration