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Note 3 - Securities - Realized Gains and Losses From Calls of Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Available for sale, proceeds	$ 126,840	$ 348,032	$ 17,287
Available for sale, book value	126,732	347,466	17,270
Available for sale, gross gain	110	1,157	17
Available for sale, gross loss	2	591	0
Available for sale, net gain (loss)	$ 108	$ 566	17
Held to maturity, proceeds			6,430
Held to maturity, book value			6,430
Held to maturity, gross gain			0
Held to maturity, gross loss			0
Held to maturity, net gain (loss)			$ 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