Note 3 - Securities - Realized Gains and Losses From Calls of Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Available for sale, proceeds	$ 17,287	$ 13,620	$ 220,520
Available for sale, book value	17,270	13,614	220,334
Available for sale, gross gain	17	10	186
Available for sale, gross loss	0	0
Available for sale, net gain (loss)	17	10	186
Held to maturity, proceeds	6,430	8,975	16,160
Held to maturity, book value	6,430	8,971	16,114
Held to maturity, gross gain		4	46
Held to maturity, gross loss	0	0	0
Held to maturity, net gain (loss)		$ 4	$ 46

Book value of available-for-sale securities that were called during the period.

No definition available.

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- 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

- 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

- 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

- 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

- 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

- Definition

The gross realized gain (loss) on held-to-maturity securities that were called during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The gross realized loss on held-to-maturity securities that were called during the period.

+ References

No definition available.

+ Details

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Namespace Prefix:	nksh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration